Download Bedrock Geology of the High Peaks Region, Marcy Massif

1
GEOLOGY
BEDROCK
of
PEAKS
HI G H
REGION,
MASSIF, ADIRONDA CKS, N.Y.
MARCY
Howard
the
w.
pa u I
W
e.
Elizabeth
Jaffe
•
L e o
Bo Jc
8 6 7
0 1111 a+
M.
Jaffe
Ha I I
LOJ
lAKe
PLAc10 N.y.
CONTRIBUTION NO. 46
DEPARTMENT OF GEOLOGY& GEOGRAPHY
(I NI VE RSITY
OF
AMHERST
'
MASSACHUSETTS
MASSACHUSETTS
N
...
~:,:
7"'1';;···,
'
~.::':.'I.
..
Plate 1. Mt. Marcy, 5344'(1629m), from the summit of Skylight Mt., 4926'(1502m),
core of Marcy anorthosite massif, Mt. Marcy quadrangle.
3
Dedicated to
Willie and Bud
friends, colleagues, and
Adirondack field geologists
A. Williams Postel
(1909 - 1966)
A.F. Buddington
(1890 - 1981)
15' quadrangles mapped in the Adirondacks
Dannemora (1951)
Antwerp (1934)
Churubusco (1952)
Hammond (1934)
Lyon Mt. (1952)
Lowville (1934)
Clinton Co.
Santa Clara (1937)
Magnetite District (1952)
Willsboro (1941)
Mooers (1954)
Big Moose (1950)
Moira (1955)
Port Leyden (1951)
Chateaugay (1956)
Saranac Lake (1953)
Loon Lake (1956)
St. Lawrence Co.
Malone (1956)
Magnetite District (1962)
Nicholville (1959)
~
Plate 2. Mcintyre Range in the anorthosite core of the Marcy Massif, showing Algonquin
Peak, 5114'(1559m), Boundary Peak, 4850'(1479m), Iroquois Peak, 4850'(1479m),
and Mt. Marshall, 4360'(1329m). Mt. Marcy and Santanoni quadrangles.
.-!:·:
.~ i
',
"~.~
.... ,
1~
:;.·~~-:·-.·,.
:· ·'i·--·,.
"'.'~~·ii»::-:~"';...,:.. ·r
·:~~:;~>~~:.~)·;~·:-~
fa~"'°-!~~'r,.':;1·
•, , --~ff
..-" '.>
, • • • ".
-.:
~
• • ::
v
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~
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• ·-
•
'
.... ,:>:;;.t.J/!P,i
•
"'
. -·~i-:~
-~t~~-
•:'+:~'.~'
''~(~~.~~~/·
.·;r·.:;. ~..i~.._..
~.•:·
Plate 3. View NE from summit of Skylight Mt. across prominent cone of Haystack Mt., 4960'(1512m), to
Giant Mt.(center horizon), 462 7' (141lm) ; from left margin to center are: Basin Mt., 4827' (1472m),
Gothics, 4376'(1444m), and Wolfj aw Mts. , 4185'(1276m), of the Great Range, High Peaks region,
Mt. Marcy quadrangle.
VI
°'
Port Kent
44°30
c
0
Q.
E
~
.c:
(.)
TUPPER LAKE
44°00
-IO
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ff)
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ft')
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, , , , , ,..l'j('.\~ !A'?llj('i'"/llj
KM
43045'
50
Figure 1. Generalized geologic map of the Marcy anorthosite massif and surrounding
areas: anorthositic rocks l~jJ
ferrosyenite-ferromonzonite gneiss ~ ,
and Grenville metasediments
• Grenville Club 1983 field trip stops 1-8,
10, 12, are in the Mt. Marcy quadrangle; stops 9A and 9B are in the Santanoni
quadrangle; stop 11 is in the Elizabethtown quadrangle.
j
7
INTRODUCTION
Location and geologic setting
The northeastern Adirondack Highlands are an impressive mountain range
dominated by a High Peaks Region embracing 43 mountains that reach above
4000 1 (1219 m) culminating in the summits of Mt. Marcy(Plate 1) 5344'
(1629 m), Algonq:uin Peak(Plate 2), 5114' (1559 m), Mt. Haystack(Plate 3),
4960 1 (1512 m), and Mt. Skylight, 4926'(1502 m), all lying in the westcentral part of the 15' Mt. Marcy quadrangle(Fig. 1 and Plate 4). Total
relief in the Mt. Marcy quadrangle is 4364'(1330 m). Coarsely crystalline, felsic andesine anorthosite locally capped by mesocratic, equigranular, sub-ophitic, biotite leuconorite(Table 1) makes up the sunnnits
of these four highest mountains. At the top of the Adirondacks, Marcy
summit is capped by a thin layer or raft of biotite leuconorite "floating" in coarse, megacryst-rich andesine-labradorite(An 48-50) anorthosite. Plagioclase megacrysts have been deflected by and appear to have
flowed around the earlier-crystallized leuconorite raft(Fig. 2). Some
of these plagioclase megacrysts show an unusual type of zoning visible
on outcrop scale as concentric blue and white layers. Blue zones are
labradorite, An 50, containing abundant oxide dust; white zones, also
An 50, are free of oxide dust but are antiperthitic, containing abundant
exsolution blebs of orthoclase(Fig. 2). Of the 43 High Peaks above 4000'
(1220 m), 34 are made up of andesine anorthosite, 7 are of highly deformed gabbroic anorthosite augen gnei~s(herein named Van Hoevenberg
Gneiss), and 2 are represented by quartz-poor, alkali-feldspar- and
iron-rich rocks, a monzonite gneiss on Armstrong Mt., 4400'(1341 m) and
a monzonite granulite on Giant Mt., 4627'(1411 m).
The Marcy anorthosite massif is delineated by a major northwestsoutheast trending lobe and a smaller north-south trending lobe coales- 2
cing to ~he south to form a heattshaped outcrop pattern covering 5000 km
(3100 mi ). In section, the major northwest trending lobe approximates
to a piano bench or slab 3 - 4.5 k.m(l.8 - 2.7 mi.) thick with two legs
or feeder pipes extending at least 10 km(6 mi.) down according to the
geophysical model of Simmons(l964), whereas Buddington(l969) favored an
asymmetric domical shape based upon extensive field mapping and other
considerations. The massif consists of a coarsely crystalline core of
apparently undeformed felsic andesine anorthosite intruded into and
thrust over a multiply deformed roof facies consisting of gabbroic noritic anorthosite, gabbroic anorthosite gneiss(Van Hoevenberg Gneiss),
and quartz-bearing ferrosyenite - ferromonzonite("quartz ferromangerite")
facies of a Pitchoff Gneiss(Table 1). Remnants of a siliceous carbonateand quartzite-rich metasedimentary sequence and associated garnet-pyroxene-microperthite granulites form discontinuous screens and xenoliths
in the roof facies. Xenoliths of any kind are very rare inside the felsic anorthosite of the core but abound in the gabbroic anorthosite of
the roof facies. Representative modes of these rock types are given in
Table 1. The heart shaped Marcy anorthosite massif is surrounded by
large bodies of the ferromonzonite facies of the Pitchoff Gneiss and the
metasedimentary sequence of a presumed Grenville age. A conspicuous
feature of the supracrustal sequence is the intimate intercalation of
00
Representative modes of anorthositic and iron-rich monzonitic rocks from the
Marcy Massif, N.E. Adirondacks, New York.
Table 1.
Qz
Kf
Plag
Opx
Cpx Gar
Hbl
Bio
Ox
Ap
Zr
-
5
92
0.5
0.5 1.5
-
0.5
+
-
46
48
-
-
1
1
+
-
38
47
-
1
+
+
86
21
2
1
+
92
16
2
3
+
85
22
Fe An(plag)
(opx) (gm) (meg)
MARCY ANORTHOSITE(CORE)
Andesine anorthosite
BA-2
Biotite leuconorite
MA-10
6
76
15
1
48
PITCHOFF GNEISS
Ferrosyenite gneiss
P0-2
2
84
4
2
4
1
2
Qz ferromonzonite gn
SC-6
9
46
25
2
9
4
2
Ferromonzodiorite gn
G0-2
2
22
57
4
2
7
1
__ 2~!~~:!!~~-~~!~~!~~!~---------~~=§----=----~---~2--~~---!! ___ !Q ____: ____: __!§ ____ §___: ___ ~~----~!---~~-VAN HOEVENBERG GNEISS
Gabbroic anorthosite gn.
"
"
II
SB-6
2.
5
53
+
9
19
4
1
-
42
41
43-51
2
6
56
2
8
9
4
-
7
G0-5
6
7
-
52
37
43
PR-1
-
1
74
4
14
-
6
-
1
+
-
44
44
48
-
16
51
3
8
16
-
-
5
1
+
73
26
-
-
+
-
MARCY ANORTHOSITE(ROOF)
Gabbroic anorthosite
ANORTHOSITE-SYENITE HYBRID
(TRANSITION ROCKS)
Microperthite granulite
JB-2
- 2 2 - 73 20 50
14
BA-5
20
- 19 25 13
- 5 4 - 63 23
------------------------------------------------------------------------------------------------------------
32
46
1
3
14
CA-17A -
18
18
7
56
-
BA-6
II
GRENVILLE or BASEMENT
Microperthite granulite
"
BA-6f
32
10
8
49
1
+
-
73
23
1
-
-
73
24
9
Sc~LE
~
l
,
'
')01
(Piaf· '7t~J'iJCr~rls
7fo-t f:e, .scale).
OvrcRoP M.qp
l>
f
Mr.MARC'j
SvMMIT
i. n
Anorf:hosi..te
Pl ar;. Megacr:Jsts
.Pio"" CJround raPt
/30th Zo>tt::$
h "iii/~ .:::: ll?T°Jo
1/-,...so
o
Figure 2. Schematic outcrop map of the summit of Mt. Marcy, showing a
biotite leuconorite raft "floating" in felsic anorthosite. Calcic
plagioclase megacrysts are deflected by and flow around the earlycrystallized raft. Mt. Marcy quadrangle.
10
Figure 3. Generalized outline map of Nain (clear) and Grenville (stippled)
provinces of the Canadian Shield and Greenland, showing location of
anorthosite bodies (ruled), An contents of plagioclase, and radiometric dates where knwon. Modified after Emslie, 1974.
:Zoo}'<.-,.,,
HAR.CY 11/JSS IF
A D I R 0 J\J DA C KS
ll??~o -SS
1Z88 .Sm-Nol.
1/:50
Pb-U
11
metamorphosed felsic igneous and calc-siliceous sedimentary sequences,
both away from as well as inside the massif. The felsic gneiss components
of this sequence may represent volcanics or sills interlayered with metasediments, both of Grenville age, or they may be remelted and remobilized
portions of a basement complex(Wiener, McLelland, Isachsen, and Hall,
G.S.A. Mem., in preparation).
Anorthosites
Massif type anorthosites cons.isting largely of cumulate plagioclase
An 40-60, accompanied by subordinate amounts of orthopyroxene, clinopyroxene, olivine, ilmenite, and apatite make up a large part of the
crust of the Nain and Grenville Provinces of the Canadian Shield,(Fig. 3,
modified from Emslie, 1974). The Adirondack Marcy massif may be considered as a southern extension of the Grenville Province across the
narrow Frontenac axis. Plagioclase cumulates(or adcumulates)in anorthosites of the Nain Province(Fig. 3) intruded about 1450 M.Y. tend to be
richer in An, 50's - 60's, than those of the Grenville Province, 12881150 M. Y. , which have An in the 40 's and low 50 's. This has led some
investigators to suggest or propose the existence of a labradorite-type
and an andesine-type of anorthosite(Isachsen, 1969). Although some dispute such a division as arbitrary and artificial, there does appear to
be a correlation between An content of plagioclase and age of crystallization; the older the anorthosite, the higher the An content. Anorthosites of the Nain Province are largely ;unmetamorphosed and commonly contain troctolite members or olivine, in addition to having higher An
contents, whereas younger anorthosites of the Grenville Province are
regionally metamorphosed and olivine-free, in addition to being lower in
An content. It is generally believed that an.orthosites of both the Nain
and Grenville Provinces intruded in an ariorogenic rifting environment
with the latter becoming overprinted by a granulite facies regional
metamorphism(Emslie, 1977 and Morse, 1983). Buddington(l969) Martignole
and Schrijver(l970), Michot(l969), and others, however, favor syntectonic
emplacement under granulite facies conditions. Because sodic plagioclase
is a more stable liquidus phase than calcic plagioclase at high pressures
(Green, 1969, 1970) it is conceivable that andesine anorthosites of the
Grenville Province such as the Adirondacks, Morin, and St. Urbain, have
crystallized at deeper levels than those of the Nain Province. The average An content of plagioclase from the Harp Lake complex(Emslie, 1977)
is 60 whereas that of the Adirondack massif is 46. The paucity of garnet
and the low color index of felsic andesine anorthosite of the core of the
Marcy massif preclude the possibility that metamorphic growth of garnet
could significantly reduce the average An content of the massif from 60
to 47; the difference is real.
The essential mineralogical makeup and compositional range of both
anorthositic and alkali-feldspar-rich rocks are conveniently represented
on the ternary diagram: plagioclase-pyroxene+oxides-microperthite+quartz
equivalent to anorthosite-pyroxenite-alaskite(Fig. 4). We recognize an
anorthositic series and a Pitchoff Gneiss made up of quartz-poor, ironrich syenitic-monzonitic-syenodioritic facies that are assumed to repre. sent separate batches of melt rather than a true fractionation series.
Figure 4. Ternary diagram: Plagioclase - Pyroxene + oxides - microperthite + quartz =
Anorthosite - Pyroxenite - Alaskite, showing variation in mineral composition with
color index for anorthositic series and monzonitic (mangeritic) rocks.
A LASK/TE
Micrope rthite+Quartz
yenite,
ranite
(Chornockite)
Mofic
Hyp.
Aug.I
~ 1LJ LJ
'LJ\.lntermed.
{ llm.
v
"
Pio
Mt.
Melagabbro Gobbro Gobbroic
g.
PYROXENITE
An(plog) Anorth.
ANORTHOSITE
3.3<
6 0(
Mofic
Fs (qpx)
56---7 4s
29
> 42
< . ··------- _
fersic
I-'
N
13
Our Pitchoff Gneiss is analogous to the "quartz mangerite series" of
de Waard(1969, 1970) and others only insofar as petrographic distinctions
are made(Fig. 4). According to a model suggested by Buddington(1969) for
the Adirondack anorthosite series, a parent gabbroic anorthosite magma
made up of 60% liquid, 40% crystals, sheds and concentrates calcic andesine megacrysts by a process of flow differentiation to form a core
facies, while a residual liquid fractionates towards increasingly more
pyroxene+oxide-rich gabbroic anorthositic compositions characteristic
of a roof or border facies; color index increases with iron enrichment
in pyroxenes as plagioclase decreases in An content(Fig. 4). By contrast, in the alkali-rich rocks of th_e Pitchoff Gneiss (="quartz mangerite series"), color index decreases with iron enrichment in pyroxenes as
plagioclase again decreases in An content. Similar, although not identical relations have been observed by Ollila(1983, in preparation) in the
Santanoni quadrangle, by Davis(l971) in the St. Regis quadrangle, and
by Jaffe et al, (1975, 1978) in the Mt. Marcy quadrangle from which the
data of Figure 4 are plotted. Ollila in the Santanoni and Jaffe and
Jaffe in the Mt. Marcy quadrangles have recorded only a small decrease
in the An content of plagioclase of felsic anorthosite and noritic
anorthosite accompanying the increase in the iron content, Fs, of pyroxenes. This An-Fs contrast, however, increases and becomes exaggerated
in more mafic mineral-rich anorthosites because of extensive garnetforming reactions, during subsequent metamorphism, which effected a
marked An depletion in plagioclase. Most of the garnet in both anorthositic and monzonitic rocks from the Adirondacks contains 15-20% of the
grossular molecule which derives entirely from the anorthite component
of pre-metamorphic plagioclase.
Geologic mapping in the High Peaks Region
Field work in the Mt. Marcy quadrangle and environs was already under
way in the mid-19th century when Emmons described the geology around the
Cascade Lakes in his Survey of the Second Geological District in 1842.
Kemp in 1898 published a geological map of part of the Lake Placid quadrangle and the northern part of the Mt. Marcy quadrangle, which the N.Y.
State Museum offered for sale at 5 cents per copy. Miller in 1918 completed a geologic map of the entire 15' Lake Placid quadrangle, and Kemp
in 1921 provided us with the first map of the geology of the entire 15'
Mt. Marcy quadrangle. We have had to wait until the 1980's, however,
for one of our trip leaders, Paul Ollila, to produce the first ever completed geological map of the relatively inaccessible 15' Santanoni quadrangle(Ollila, Jaffe and Jaffe, 1983) to the west of the 15' Mt. Marcy
quadrangle. In the 1960's, Crosby remapped a large part of the 15' Lake
Placid quadrangle and suggested the existence of nappe structures in the
region. Many geologists were, and continue to be, skeptical of the presence of nappe structures in the High Peaks Region although McLelland and
Isachsen(l980) clearly documented their occurrence in the southern Adirondacks. Northeast of our trip area Olmsted and Whitney are remapping the
northern and southern parts of the Ausable Forks 15' quadrangle originally
mapped by Kemp in 1925. To the south of the High Peaks Region Turner is
remapping part of the 15 1 Schroon Lake quadrangle, originally mapped by
Miller in 1918. The area to the east, the 15' Elizabethtown quadrangle
which we shall visit on this trip, was mapped by Kemp and Ruedemann in
......
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/
---- ---- --.....
,
--- -,,,,..
/
53~"
~,A_:.'/'
,,..... ~,,,.
Ma. rrcy
/
,,,.:---
~
-+
..,_
y
./....
gA
x
y
Q t:tb.b r-01c
• '
y
><
y._
a
x
)\
x
-
x
)\
--t;e..
vio
x_
gA
)('
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..,.t;-f,,oSJr
x_
x
x
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x
)(
x.
/A~''°''
--.;..,,_::_
-\"'
x
-fFigure 5. Schematic NE-SW geologic cross-section viewed from Cascade Summit, showing the Great
Range Nappe of the High Peaks Region. Anorthosite~of a core facies is thrust over
intensely deformed gabbroic anorthosite augen gneiss l;.'I and ferromonzonite gneissIDIIIIII
of the roof facies. Mt. Marcy quadrangle.
15
1910 and was partly remapped by Walton in the 1960's, and by oeWaard on
Giant Mt. in 1970, and in an area to the east by Gasparik in the 1970's.
Why then, remap? Although the rocks have changed but little, new
concepts, new techniques, and a plethora of laboratory experimental data
unavailable until recent years have combined to change the way in which
geologists examine, correlate, and map outcrops. For example: Emmons
after studying the marble outcrops above Cascade Lakes, concluded, in
1842, p. 228-9, that they "provide undoubted evidence that the limestone
is an injected mass, or in other words, a plutonic rock". Miller in his
geology of the Lake Placid quadrangle strongly denied the existence of
compressive folding in the Grenville gneiss in 1916 and again in 1918,
when he wrote that "If the Grenville and accompanying great intrusives
had been subjected to compression severe enough to develop the foliation,
is it not remarkable that the stratification surfaces have never been
obliterated and cleavage developed instead, and also that the stratification and foliation are always parallel? Also, unless we assume intense
isoclinal folding, so that mineral elongation could everywhere have taken
place essentially at right angles to the direction of lateral pressure,
the parallelism of stratification and foliation cannot be accounted for
by crystallization under severe lateral pressure. But the Grenville
strata were never highly folded •..•••••• Grenville foliation was developed •••. in essentially horizontal strata under a heavy load of overlying material ••• under conditions of static metamorphism"(Miller, 1918,
p. 66). We, in the 1970's and 'SO's can decipher at least four or five
periods of folding of these same Grenville strata. Kemp, mapping in the
same years as Miller, had more insight, and was indeed impressed with
the evidence for folding in marble and syenite lithologies and, in 1921,
writing of the Grenville marbles, noted "No great section, however, is
free from included masses of silicates or of fragments of the wall-rock
or of intrusive tongues torn off in the great compression to which the
region has been subjected." With regard to the gneisses, Kemp wrote
"A candid observer cannot, however, disguise from himself the possibility
that old Grenville shales may, under extreme metamorphism, assume a mineralogical composition not appreciably different from the acidic phases of
the syenite series."(Kemp, 1921, p. 17). Thus, at the turn of the century,
Kemp envisioned the anatexis of the syenites of the Pitchoff Gneiss that
we will visit on Stop 8.
Geologic mapping by Jaffe and Jaffe during 1970-1982 in the Mt. Marcy
quadrangle, and by Ollila during 1978-1982 in the Santanoni quadrangle
has delineated the existence of thrust slices of sizeable dimension. The
Jaffes recognize and have delineated a Great Range anorthosite thrust nappe
(1977) in the Mt. Marcy quadrangle that may be correlative with the JayWhiteface anorthosite nappe described by Crosby(l966) from the Lake Placid
and Ausable Forks quadrangles to the north and northeast of the Marcy area.
Mineral lineations and fold axes indicate an arcuate trending mappe transported to the north and northeast from a root zone in the High Peaks
region. This nappe is well exposed on the eastern slope of the main summit of Basin Mt.(Plate 3). Here, the Basin Thrust(Fig.5) separates the
overriding, undeformed, coarsely crystalline andesine anorthosite of the
core of the Marcy massif from the underlying highly deformed section of
......
O'\
0
145
0
~
•
•
•
•
PIG
11
I00
11
11
A
PIG 00l
11
•
A
®
®
•
O
A
_.
a
!.
c
D
•
PIG lomelloe
in
c
AUG host
FfJa
~1-Jo-S
e
e-..- R B <3 a.
•®. . ••.
•
\jr\ ... ~
W"r\-~'YY)
fl
.
•
®
•
~KV-3a.
®
0
0
8
0
65
0
G,.'\,, ~ i== ... .:5
f'"Yv6----
a
'0 - cP
..,,,...
~B-k
(f)oO~ 0
PIG lamellae
in
AUG host
10
Belchertown
Cortlandt
Hudson Highlands
Adk. Anorthosite
Adle. Qtz. Mangerite
Bjerk rem- Sogndal
Flaks tad-y
600° EXSOLUTI ON ISOTHERM
$
o
c,)o
c9o o \1_o
f O'
20
30
40
IOO(Fe• 2 +Fe'* 3.+Mn) I (Fe+
50
2
60
70
80
90
+Fe• 3 +Mn +Mg) (host augite)
Figure 6. Diagram relating the (Fe2+ + Fe3+ + Mn)/(Fe2+ + Fe3+ +Mn+ Mg) ratio with angle
of intersection of two sets of pigeoni te exsolution lamellae ( '001~'100') in host
augite. Exsolution angles are for a 600°(metamorphic) isotherm. After Jaffe et al.,
1975.
Wt~~(,~
t-A~ss\r
1
a
c
.. J o-S
----
e
/_,........
/
/
........................
:::> '2 0°
<!>'
//
<(
=
••
0
a. '1.u9
-
<!>
a..
105°
e\.ii~
9
S j).
Pig
I.
5
-.Jf_ 1.-0'
0
fe aug
=
GABBRO
w Pi g Pz. I le.
115
~
0
0
0 0
0
> a ?i.9
Q
100 O't:
c.·"' f c;..i- a.<19
\'1-i'c. i-10 \'
<a
0
~roo_,
o o
P' 9
I
L_
:,.
105°
H r G H Pl G
·
50
\00 ( Fe 2 •
0
"1
.
a a.c(9 -a
/:::>'9
800°C
30
I
l
•
____-- ...... ~]:! Pigl[• 600°C
110°
MANGERITE
• BUSHVELD
....• •••••• •
<. rr5°
0
0
ANORTHOSI TE 1_1
0 QUARTZ
•••
u
-I
EXSOLUTION ANGLES OF
PIGEONITE LAMELLAE IN HOST AUGITE
11
PIG "001
A C vs T. and fe 0 ug
Fe~•+Mn)/
70
I
C zjc
90
100°
(Fe 2 .. -tFe 3 •-+Mn ... Mg)
Figure 7. Diagram relating the (Fe2+ + Fe3+ +Mn+ Mg) ratio with angle between '001' pigeonite
exsolution lamellae and c-axis of host augite. Both low temperature(metamorphic)(Jaffe
et al., 1975) and higher temperature exsolution patterns are shown(Robinson et al., 1977).
I-'
.......
18
quartz-bearing ferromonzonite gneiss(Pitchoff Gneiss) intimately infolded
with gabbroic anorthosite augen gneis.s(Van Hoevenberg Gneiss) that make
up most of the roof facies of the massif. The summits of Saddleback,
Gothics, Armstrong, and Wolfjaws Mts. are all in roof facies (Fig. 5).
That regional metamorphism took place at high pressure, in the range
of 8-10 kb at about 800°, is indicated by the occurrence of orthoferrosilite, Fs
+ quartz, and the absence of any vestiges of fayalite in the
95
ferrosyenite facies of the Pitchoff Gneiss. on Pitchoff Mountain, and in
similar rocks from Scott's Cobble and the Great Range(Jaffe et al, 1975,
1978). Because recent sm147-Ndl43 age dating yields 1288 M.Y. for the
age of magmatic crystallization of the Marcy anorthosite(Ashwal and Wooden,
1983, in press) and older Pb-U age dating by Silver(l969) yield about
1130 M.Y. for crystallization and about 1100-1020 for metamorphism, the
Grenville orogeny may have spanned as much as 200 M.Y. and it is difficult to fix the peak of metamorphism with a specific thermal or tectonic
event in this time span.
Anglesof exsolution of metamorphic pigeonite lamellae with the ccrystal axis of host augite decrease regularly with increasing Fe/(Fe+Mg)
ratios of the host in all members of the anorthositic series, and the
ferrosyenite-ferromonzonite series(Pitchoff Gneiss) indicating a pervasive low temperature metamorphic equilibration in the range of 5007000, Fig.6 and Jaffe et al, 1975. These subsolidus metamorphic exsolutions of pigeonite in host augite show an orientation pattern, abundance,
and size distribution very similar to those described by Jaffe, 1972 and
by Jaffe and Jaffe, 1973, from Proterozoic gneisses from the Hudson
Highlands. These have now been identified in all granulite facies terrain rocks of appropriate bulk composition. They differ markedly, however, from subsolidus igneous high temperature(800-1050°) pigeonite exsolution textures seen in host augites of rocks from the large layered
intrusions such as Bushveld, Skaergaard, and Nain, and indeed from ferrosyenite-ferromonzonite (ferromangerite) facies of the Pitchoff Gneiss of
the Marcy quadrangle(Ollila and Jaffe, in preparation). The latter show
much greater solid solution, hence coarser and more abundant exsolution
lamellae, and, most important, a different orientation pattern of lamellae from that observed in the much lower temperature metamorphic granuli te facies rocks (Fig.7 and Robinson et al, 1977). Textures of relict
high temperature igneous pigeonite lamellae in host augite and of coarse
augite lamellae in inverted pigeonite are preserved in some of the anorthositic and syenitic rocks even though these have been overprinted by the
pervasive low temperature metamorphic exsolution pattern. The discovery
of such relict high temperature lamellae in pyroxenes with Fs>90 in the
high iron syenite-monzonite series indicates that these rocks were
molten at temperatures on the order of 850° or greater and pressures of
8 kb or greater(Lindsley, 1983) as no traces of a potential earlier fayalite quartz assemblage are to be found(Ollila and Jaffe, in preparation).
Heating of a depressed segment of Grenville supracrustal rocks containing
biotite, hornblende, carbonates and felsic volcanics by anorthositic
melts at 28 km(l7 mi) could produce the quartz-poor, alkali- and ironrich anatectic melts in the presence of a co2-rich fluid phase. According to the data of Wendlandt(l981) such melts would become progressively
richer in feldspar and lower in quartz compared to lower pressure melts.
Mol. % En
70
65
60
55
Cl)
50
45
40
35
30
MARCY ANORTHOSITE SERIES
0
Cl
ORTHOPYROXENE-PLAGIOCLASE
RELATIONS
Q)
E
55
(orthopyroxene)
)(
45
a.
0
Q)
..c
(.) •
50
">
- <·- -
·c
Cl)
Gabbroic anorthosite
gneiss - O
0
50 ";
Cl)
0
0
I
0
o
I
0
<l'.
0
·ei 45
g
55
0
a.
a.
60~
~ 40
0~
Biotite leuconorife - A
~
Melagabbro -
w
~ 35
0~
+
0
65~
z
a::~
<l'.
{!)
30
70
I
I
30
35
40
45
Mol. % Fs
50
55
60
65
70
( orthopyroxene)
Figure 8. Diagram showing orthopyroxene-plagioclase relations in anorthosite series
rocks. Tie lines connect disequilibrium assemblages of: giant Al-rich orthopyroxene + plagioclase megacrysts with their analogues in host anorthosite, and
megacrysts and matrix in gabbroic anorthosite, gabbroic anorthosite gneiss, and
oxide-rich melagabbro. Gamet "isograd" is at 45% Fs in orthopyroxene.
Mt. Marcy quadrangle.
......
\C
20
Ensuing magma mixing with anorthositic melts carrying calcic andesine
megacrysts might produce the hybrid or transition rocks found in the
Adirondacks near ferro-monzonite-anorthosite contact zones.
Equilibrated and tmequilibrated orthopyroxene and plagioclase assemblages also occur in the anorthositic series(Fig. 8) and may result from
magmatic convection, multiple intrusion, and perhaps from metamorphic
processes such as garnet-, hornblende- and scapolite-producing reactions.
Thus, Al-rich bronzite megacrysts, Fs29, accompanied by labradorite,
An5 5 , occur in host gabbroic or felsic anorthosite containing hypersthene, Fs40 and labradorite, An 50 (Fig. 8). Some of these giant Al-rich
bronzites contain spectacular, doubly terminated exs.olution lamellae of
pyrope-rich garnet in addition to lamellae of bytownite-anorthite. Such
megacrysts are obviously exotic xenocrysts either convected into crystallizing anorthosite magma from a deep crustal or mantle source(Emslie,
1975) or intruded into crystallized anorthosite from a deep crustal or
mantle source(Jaffe et al, 1983, and in press).
Apparently tmdeformed andesine anorthosite commonly shows a welldeveloped magmatic alignment of euhedral, 3-6 cm (1.2-2.4") long, conspicuously Carlsbad twinned, dark blue gray megacrysts of calcic andesine,
most often An46-48 lying in a white matrix of plagioclase of identical
composition. Gabbroic or noritic anorthosite, when undeformed, shows
ophitic fabric and contains one plagioclase, An 45 _ • Deformed and
55
crushed gabbroic anorthosite is often hornblende-, scapolite-, or garnetbearing and may contain plagioclase megacrysts of varying An content. A
single hand specimen of gabb~oic anor'thosite from Cascade summit in the
Mt. Marcy quadrangle contains tmzoned megacrysts of composition: An 47 ,
An48.5' An4 9 . 5 , and An54. 5 (Fig. 8), all in a granulated matrix of finegrained, zoned plagioclase, An 41 -'-4J• Gneissic varieties of anorthosite show similar differences in°composition of megacryst and matrix
plagioclase composition(Fig. 8).
Garnet may appear in all anorthositic and monzonitic(mangeritic)
rocks when the orthopyroxene composition exceeds Fs45(Fig. 8) provided
that ilmenite and/or magnetite is present. It is, however, always absent
from an early crystallizing ophitic leuconorite which is low in oxides
and contains biotite. Garnet occurs in three principal textural types:
1) in anorthositic rocks as coronas made up of small euhedra surrotmding
ilmenite, orthopyroxene, or even apatite, 2) in quartz monzonite(mangerite)
gneisses(Pitchoff Gneiss) as a finer grained aggregate of anhedral sym plectite with vermicular quartz, as. well as in single euhedra in contact
only with plagioclase, and 3) in anorthositic rocks as anhedral grains or
elongate rods nucleating inside of and replacing zoned sodic andesine
along albite twin lamellae. Formation of garnet with increasing iron enrichment in orthopyroxene accompanied by opaque oxides supports similar
findings by McLelland and Whitney(l977) and by Martignole and Schrijver
(1973) for both the Marcy and Morin complexes. They believe that garnet
forms in these rocks due to a retrograde cooling at pressure whereas the
present authors agree with Buddington(l969) who favored a regional metamorphic origin for most of the garnet. In view of the different textural
varieties observed, it is probable that more than a single process is
responsible for nucleation of garnet.
21
Further complicating the metamorphic history of the region is the
presence of monticellite in silicated marbles in contact with quartz
syenite gneiss infolded with gabbroic anorthosite on Cascade Slide and
elsewhere in the Mt. Marcy quadrangle where the assemblages: calciteforsterite-diopside-magnetite-spinel, calcite-forsterite-monticellitediopside-spinel-aridradite and calcite-forsterite-diopside-monticellitespinel-idocrase were reported by Tracy, Jaffe and Robinson, 1978. Jaffe
in Baillieul(l976) also reports the second occurrence of the rare calcium
borosilicate, harkerite, at this locality, and Valley and Essene, 1979,
1980, report the occurrence of futermanite, from the same locality. Wollastonite occurs in calcareous quartzites and siliceous marbles at
several localities in the quadrangle and may occur in proximity to monticellite-bearing assemblages. One additional sample contains the assemblage calcite - quartz-diopside-grossular without wollastonite. The
presence of monticellite in rocks metamorphosed at high pressure may be
due either to a low activity of CO~ in the metamorphic fluid(Tracy et
al, 1978), a complex metamorphic history of regional superposed on
earlier contact metamorphism, or marked variations in the co 2 and H20
fluid compositions during regional metamorphism(Valley and Essene,
1980).
Retrograde metamorphic effects are seen mostly in highly deformed
rocks where communication with water, halogens and carbon dioxide was
possible, these being available in, and supplied by, the Grenville
supracrustal sequence. In deformed gabb~oic-noritic anorthosite roof
rocks, hornblende rims augite, often with the development of quartz;
scapolite replaces andesine in fracture zones, and garnet is at least,
in part, later than hornblende. Prehnite, pumpellyite, chlorite,
calcite, sericite, and albite occur in very late fractures that postdate scapolite-hornblende assemblages. Microperthite and ferroaugite of
syenitic-monzonitic gneisses connnonly show two and often three different sets of exsolution lamellae attesting to repeated subsolidus reequilibration. Extensive retrograding is found in mylonitic contact
zones of the larger pre-metamorphic garnetiferous gabbro dikes that
intrude the andesine anorthosite of the core facies of the Marcy massif.
An example of these is the 75'(23 m) wide Avalanche dike in Mt. Colden,
in which hornblende replaces augite, serpentine replaces hypersthene,
and scapolite replaces plagioclase, and where slickensided joints are
coated with mylonitic smears of cryptocrystalline quartz, serpentine,
and calcite(Jaffe, 1946). Post-metamorphic diabase dikes show total
retrograding of all mafic minerals to fine-grained aggregates of calcite,
chlorite, talc, and magnetite. The geologically much younger camptonite
dikes show only slight serpentinization of olivine phenocrysts and extensive deuteric alteration of clinopyroxene only in very narrow zones
that immediately border spherical calcite+sodic plagioclase-filled
amygdules(Jaffe, 1953 and Fig. 10).
Stratigraphic succession and structural history of the Mt. Marcy
quadrangle.
Geochronologic setting:
An extensive geochronologic investigation of syenitic and anorthositic
22
Table 2.
Stratigraphic succession and tectonic history-Mt. Marcy quadrangle.
CORE FACIES
DIKE INTRUSION
Camptonites
NE-SW
DIKE INTRUSION
Diabases
NE-SW
BLOCK FAULTING
NE-SW
OPEN FOLDING Axes
NE-SW,NNW-SSE
Map symbols are not distinct
from those of roof facies
Metagabbro dikes
NW-SE
INTRUSION
Gabbro includes melagabbro,
oxide-rich gabbro, pyroxenite,
and anorthositic gabbro
INTRUSION
GABBROIC ANORTHOSITE includes
gabbroic and noritic anorthosite,
c. I. 10-35
INTRUSION
ANORTHOSITE - andesine ± hypersthene, C.I. < 10, containing
rafts of biotite leuconorite
ROOF FACIES
OVERTHRUSTING
Map symbols not distinct from
those of core facies above
RECUMBENT FOLDING
SW-NE
Axis
NW-SE
PITCHOFF GNEISS - syenitemonzonite gneiss with opx Fs 80_ 95
ANATEXIS AND INTRUSION
VAN HOEVENBERG GNEISS - .gabbroic
anorthosite augen gneiss with
layers of oxide-rich gabbro and
pyroxenite
GABBROIC ANORTHOSITE - hornblende
gabbroic and noritic anorthosite
Gabbroic anorthosite with prominent
garnet coronas
Percalcic anorthosite contaminated
with calc-silicate and amphibolite
INTRUSION
ANORTHOSITE - andesine anorthosite,
C.I. < 10, enclosed in block structure
INTRUSION
RIFTING
RECUMBENT FOLDING
Axis
NW-SE
E-W
MICROPERTHITE
GRANULITE - garnetorthopyroxene(Fs60_80)-clinopyroxeneplagioclase-K-feldspar granulites,
C.I. 5-50, of igneous parentage
(sills or volcanics)
RECUMBENT FOLDING
Axis
E-W
GRENVILLE SUPRACRUSTALS UNDIFFERENTIATED
including calc-silicate, quartzite,
marble, pyroxene microperthite granulite,
amphibolite, hornblende granite gneiss,
scapolite gneiss
SEDIMENTATION AND VOLCANISM
23
rocks of the northeastern Adi.rondack.s. was carried out by Silver(l969)
using Pb-U isotopic compos:itions. of zircon. Because of the paucity of
zircon in anorthositic rocks and its abundance in the syenitic gneisses,
the bulk of Silver's data may be more applicable to the metamorphic,
rather than to the igneous events. Nevertheless, Silver obtained values
of 1130 x 106 ~rs, believed to represent anorthositic intrusion, and
1100-1000 x 10 yrs. for the metamorphic(Grenville orogeny) event. He
believed, with Buddington, that these events were largely synchronous
or zsntectonic. Ashwal and Wooden(l983, in press) recently used sml47
Ndl . isotopic dating to establish a 1288 x 106 yr plagioclase -pyroxene
isochron believed to represent a crystallization age for andesine anorthosi te of a core facies, and 995-973 x 106 yr from garnetiferous oxiderich melagabbro dikes that intruded anorthosite, and are thought to represent the prograde metamorphic(Grenville orogeny) event. If the recent
Sm-Nd dates are more correctly interpreted, we are dealing with an interval of about 300 x 106 yr. between the crystallization of a core
anorthosite and the peak of the Grenville orogeny. How much younger the
melagabbro dikes are than the anorthosite they intrude has not been established. Nonetheless, these Sm-Nd data would appear to cast doubt upon
a syntectonic origin for intrusion and deformation. Because no dates
older than 1300-1350 x 106 yr have been reported for Grenville province
rocks, the Sm-Nd dates leave but 12 - 67 x 106 yr for sedimentation, induration, and two periods of recumbent folding prior to anorthosite intrusion(Table 2). The geochronologic data and the field relations(Table
2) thus indicate that the Grenville orogeny began before the intrusion
of anorthosite, at perhaps 1300 x 106 yr, then culminated at about 1000
x 106 yr, and subsequently di.miniShedto perhaps 975 x 106 yr in a retrograde phase. The duration of the Grenville orogeny is thus on the order
of 350 x 106 yr and should not be thought of as the 1000 x 106 yr event
so often recorded in the literature.
Field relations:
Roof facies (Table 2)
In the Proterozoic, perhaps about 1350 x 106 yr, a succession of
sedimentary and volcanic rocks here referred to as the Grenville Series
(Table 2, gvu) was deposited in a marine environment. These were limestones, siliceous carbonate rocks, sandstones, and marls, with intercalated mafic and felsic volcanics or sills(Table 2, sgb). These strata
were then folded and metamorphosed once and possibly twice. The direction of the axes of these earlier folds is obscure, but may be east-west
like those of the early folds in the southern Adirondacks(McLelland and
Isachsen,1980).
Anorthosite of the present roof facies(Table 2, a, ga, gac, ap) was
then intruded into these folded layers along a NW-SE rift, which is defined by the long axis of the principal lobe of the anorthosite massif
(Fig. 1). This may have occurred at a depth of 35 km(21 miles) or perhaps may have taken place at a shallower level(Valley and O'Neil, 1982),
with the anorthosite-Grenville complex then being buried, depressed, or
dragged down to 35 km(21 miles), where it was metamorphosed.
The anorthosite of the roof facies did not intrude as a single body
24
which, on cooling, developed a chilled border and coarse-grained interior.
Instead, there were multiple pulses of intrusion of anorthositic magmas.
with different plagioclase:pyroxene:ilmenite ratios. Some were liquid,
and others were partly or almost wholly crystalline. Xenoliths of one
type of anorthosite in another are common, as are xenoliths of Grenville
series rocks with high melting points. Dikes of anorthosite occur in
different types of anorthosite. Alkali-rich members of the Grenville
series, with lower melting temperatures(Table 2, pog) were partly to completely melted by the invading anorthosites, to intrude the solidifying
anorthosites in their turn.
The early anorthosite intrusions, the Grenville series rocks, and
their hybrids, were isoclinally and recumbently folded together along
an east-west axis more or less parallel to fold axes developed during
the two early phases of folding. The Pitchoff and Van Hoevenberg gneisses
(Table 2, pog and vhg) were formed at this time.
Field relations: Core facies (Table 2)
The core facies consists predominantly of relatively undeformed
felsic andesine anorthosite intruded by smaller bodies of gabbroic and
noritic anorthosite, and by dikes of pyroxenite, garnet+oxide-rich melagabbro, and garnetiferous gabbro, also relatively undeformed. In the
Santanoni quadrangle, gabbroic anorthosite clearly intrudes felsic andesine anorthosite along a NW-SE trend with two short lobes oriented NESW. In the Mt. Marcy quadrangle, NNW-SSE-trending pyroxenite dikes intrude felsic anorthosite on Roaring Brook in the easternmost part of the
map area(Plate 4). These dikes and the anorthosite they intrude were both
fragmented and healed by the later intrusion of gabbroic anorthosite.
Pre-metamorphic garnetiferous gabbro dikes(Jaffe, 1946) commonly intrude
the core anorthosite along NW-SE fractures.
Typical felsic anorthosite of the core facies has a protoclastic
texture characterized by angular to sub-rounded, dark blue-gray andesinelabradori te megacrysts in a white, granular andesine-labradorite matrix;
both megacryst and matrix have the same An content; commonly 46-48, but
occasionally greater than 50. The core anorthosite has retained a characteristic alignment of plagioclase megacrysts interpreted to represent a
flow foliation. Proof of its igneous origin can be found near contacts
with biotite leuconorite rafts where the plagioclase megacrysts of the
anorthosite have been deflected by and appear to have flowed around the
rafts (Fig. 2). Foliation of megacrysts. in the core anorthosite is generally steep to vertical, but otherwise randomly oriented; notable exceptions to this are at the top of Haystack and Basin Mts., where they are
flat to horizontal. Intrusive contacts of core anorthosite with rocks
of the roof facies have nowhere been observed. Instead, near the top
of Basin Mt., undeformed core anorthosite has overriden intensely deformed
rocks of the roof facies(Fig. 5). This contact, named the Basin Thrust,
is exposed at the 4760'(1451 m) level on the eastern slope of Basin Mt.
Here, the underlying and deformed ferromonzonitic gneiss of the roof
facies is shattered and sheared from 4740'(1445m) to 4760 '(1451 m).
Immediately above this shear zone lies relatively undeformed, coarsely-
25
crystalline, megacryst-rich, felsic andesine anorthosite which forms
the rest of Basin Mt. and all of the highest Adirondack Mts. innnediately
to the west(Mts. Haystack, Marcy, Skylight, Colden, Algonquin, etc.,
Plate 4). To the east of this contact, all rocks of the roof facies,
including anorthositic members are intensely deformed. The core has
been thrust over the roof.
Field relations sunnnarized in Table 2 indicate that two periods of
folding of Grenville strata were followed by intrusion of anorthositic
melts and in turn, by syenitic-monzonitic melts, of a roof facies. All
of these rocks underwent pervasive recumbent folding followed by overthrusting of a core anorthosite facies as nappe structures. The actual
mode of emplacement of the anorthosite of the little-deformed core facies
remains an enigma and any of the following scenarios are possible:
1) The anorthosite core facies was crystallized at considerable
depth and thrust over the anorthosite-syenite-Grenville complex of the
roof facies after the peak of the Grenville orogeny.
2) The anorthosite core facies was emplaced into or over the roof
facies either by faulting or by diapyric rise of an essentially crystallized mush. A major orogenic compressive event followed during which
the roof facies was recumbently folded and the core facies subsequently
thrust over the roof facies. During this event the rigid, homogeneous
core facies may have functioned as a battering ram.
3) The anorthosite of the core facies was intruded late, and more
or less syntectonically, into the same NW-SE rift environment as the
roof facies anorth<Bite, induced folding in the latter, and was eventually
thrust over the latter. Here the waning stages of an extensive rifting
event would be the harbinger of a major compressive event.
On the basis of field relations alone we cannot discriminate among
these hypotheses.
The extent to which the undeformed, but sheared and garnetiferous
anorthosite of the core has been subjected to a granulite facies regional
metamorphism may be debated. Much of the evidence rests upon the extensive crushing of plagioclase megacrysts, the common occurrence of garnet
in reaction rims between ore or pyroxenes and plagioclase in granulation
zones, presence of pervasive low temperature(500-600°) exsolution textures
that have supplanted relict higher temperature(800-1050°) exsolution textures in pyroxenes(Figs. 6 and 7), and upon the geochronological evidence
discussed above. Although the lower temperature pyroxene exsolution
textures are often ascribed to metamorphic reequilibration under granulite
facies conditions, they need not be so derived. They may alternatively
be due to the slow cooling of magmatic pyroxenes in host rocks held between 500 and 800° for a long duration at pressure. If the formation of
garnet in anorthositic rocks is caused by retrograde cooling at pressure
(Martignole and Schrijver, 1973, McLelland and Whitney, 1977), the crushing of plagioclase megacrysts by protoclasis, and the low temperature
pyroxene exsolution textures by slow cooling from 700-800°, one may question whether the anorthosite of the core facies was indeed post-orogenic
or pre-orogenic. The preservation of flow foliation and a 1288 x 106 yr
Sm-Nd date ascribed to crystallization of core anorthosite may suggest
26
post-orogenic emplacement; the larger number of isotopic ages in the range,
973-1100 x 106 yr sugges.t a high grade, pre-orogenic event. Melagabbro
dikes intrusive into the core anorthosite are dated at 950-995 x 106 yr
but the interval between the anorthosite crystallization and dike intrusion is not known.
We do not favor a post-orogenic emplacement of the core anorthosite
but wish to call attention to an alternative hypothesis.
At some time after the core anorthosite was emplaced, by whatever
process, open folds developed along NNW-SSE axes in the Mt. Marcy quadrangle, and NE-SW axes in the Saranac and Santanoni quadrangles. This
folding affects both the earliestanorthosite-Grenville series complex or
roof facies, and the late, overthrust an:orthosite or core facies.
Block faulting, principally NE-SW in direction, began in the Precambrian and continued at least through Middle Ordovician. The Mt. Marcy
quadrangle may be divided into three principal fault blocks.: the Pitchoff
Block, NW of the Cascade Lakes.-Henderson Lake fault is the most downdropped;
the Mt. Marcy Block, in the center of the quadrangle, and north of the
Ausable Lake fault which was downdropped less; and the Dix Block, south
of the Ausable Lakes fault zone, is the least downdropped. The northsouth Keene Valley fault zone is probably later than the Ausable and Cascade-Henderson Lakes faults.•
Post-metamorphic dikes of Late Precambrian or Paleozoic age and camptonite dikes of Cretaceous age, both intruded in NE-SW fractures.
1983 FRIENDS OF THE GRENVILLE FIELD TRIP STOPS
The 1983 Annual Friends of the Grenville Field Excursion will make
12 stops, 10 in the 15' Mt. Marcy quadrangle, and 1 each in the 15'
Santanoni and Elizabethtown quadrangles to visit representative outcrops
of anorthositic rocks, ferrosyenitic-ferromonzonitic gneiss, and Grenville
series rocks. The 12 stops are located by number on Plate 4, the topographic map bound in the back cover of this guide. The 12 stops are
also telescoped in Figure 1, a generalized geologic map of the Marcy massif.
Because most of the outcrops are on state land and preserve an often
delicate rock record of some 1350 x 106 yr, it is requested that no
hammers be used on outcrops; samples should be collected from rubble.
It is, in fact, a violation of New York State law to collect samples of
rock on state land without express. permission.
·
CAUTION: Parties of 30-60 people on trails with loose rubble and on
narrow and sometimes precipitous ledges may generate hazards. Please
exercise caution, particularly on s.tops: 2, 4, 8, and 9.
27
Stop 1.
The MARCY ANORTHOSITE-CORE FACIES-MT. JO
Mt. Jo, elev. 2876' (877 m.), which lies. just north of Heart Lake,
elev. 2178'(664 m), and is the only mountain owned by the Adirondack
Mountain Club, offers a full range of anorthosite lithologies for s.tudy
of the primary igneous textures as.sociated with the core of the Marcy
anorthosite massif. Although the massif has been metamorphosed under
granulite facies conditions of about 8 Kb and 800°c during the 1100 M.Y.
old Grenville orogeny, the homogeneity and rigidity of the core has permitted preservation of many of the textures associated with magmatic
emplacement such as primary flow structure expressed by aligned andesine
megacrysts in andesine anorthosite, and ophitic texture in noriticgabbroic anorthosite, and in norite pegmatite. A well-developed primary
magmatic foliation commonly observed in the felsic anorthosite core of
the Marcy massif is formed by the rude to perfect parallelism of: (010)
crystal faces, conspicuous, simple, but broad Carlsbad twin slabs, and
less obvious albite twin lamellae of the calcic andesine-labradorite
megacrysts. This megacryst foliation, although often strongly developed
on a single outcrop, may vary appreciably in azimuth over a large outcrop pavement. Frictional res.istance, convection, and impedance by crystallized masses of leuconorite may combine to produce the various foliation patterns observed. Extensive size-reduction of plagioclase megacrysts may often result from intrusion tectonics or protoclasis, rather
than from regional metamorphism or cataclasis.
There are two trails to the sununit of Mt. Jo; we will climb the
mountain via the Short Trail and return via the Long Trail. Our 700'
(213 m) climb takes off from the N.E. corner of Heart Lake and climbs at
a moderate grade to the trail junction at 0.2 mi (0.32 km). Bearing
right, up the Short Trail, we will pass our first low pavement outcrop
at 2300'(701 m). Near 2400' (731 m), the trail narrows and a prominent,
west-facing vertical wall of anorthosite rises abruptly. This first
cliff(sample locality l-Jo-14, Fig. 9) consists of very crushed felsic
or andesine anorthosite with a megacryst percentage(M.I.)=10-30 and a
color index(C.I.)=5. Calcic andesine megacrysts average 3 x 0.6 cm;coarse
megacrysts, 6 x 3 cm, are sparse and markedly anhedral because of granulation. Occasional 30-60 cm ovoid clots of coarse andesine and smaller,
broken megacrysts lie in a fine-grained matrix, often noritic. Garnet
and ilmenite tend to be associated with the coarser andesine anorthosite
component, whereas biotite may be found in the finer noritic component.
Although the extensive fracturing and abrasion of the megacrysts could be
interpreted as metamorphic in origin, we believe it occurs because of
intrusion protoclasis produced during a stage when single megacrysts and
clots of early-formed megacrysts mingled with, and were embayed and forcibly injected by, residual leuconoritic liquid.
Megacryst foliations over most of Mt. Jo trend near E.-w. and dip
nearly vertically, but those in fractured areas and in areas near leuconorite inclusions often show gentle dips. On this fractured face of the
cliff, a gently north-dipping foliation is apparent, but not representative, and conclusions regarding megacryst orientation are best deferred
until we examine pavement outcrops near the summit.
28
N
\
-- -
0
0.1
0.2
......
1-~~--1~~~
o
-0.!!J:.L~
0.3
-o.6 miles
~~~-'-~~---0~~~..A..~~~~
lm
T'igure 9. Map of Adirondack Mountain Club property, showing Heart
Lake, Adirondak Loj, and geologic locations on Mt. Jo. Mt.
Marcy quadrangle.
29
At about 2700'(823 m) near a good lookout, south, to Heart Lake, at
locality I-Jo-lO(Fig. 9) a garnetiferous gabbroic anorthosite(C.I.=15)
shows garnet, ilmenite, and pyroxenes., with plagioclase megacrysts showing a N55E80N foliation. As we scramble up the main sunnnit, near 2860'
(872 m) we see bare pavement outcrops that show coarse andesine anorthosite(M. I.=45, C.I.=5), with 4-7 x 1-2 cm(l.6-2.8 x 0.4 x 0.8') plagioclase tablets defining a marked N80W90 foliation. Characteristic coronas
of garnet around ilmenite and pyroxene may be seen in the matrix and in
fractures in megacrysts. A bit further along we encounter a small ovoid
inclusion or raft of biotite leuconorite "floating" in the felsic anorthosite. These rafts have leuconoritic-leucogabbroic composition, C.I.=
20-25, and subophitic texture(see modes, Table 3). The rafts always
contain biotite but no garnet whereas the enclosing host anorthosite
has C.I.=only 5, but always contains garnet and lacks biotite. Andesine
megacrysts flow around and were deflected by the earlier crystallized
leuconorite raft(Fig. 2). On the summit at 2876'(877 m), at locality
I-Jo-8,(Fig. 9), the two rock types are in a less well defined relation.
The summit rock consists of a 1-3 1 (30-90 cm) layer of biotite leucogabbro
containing megacrysts of andesine which show the same foliate orientation
seen in the adjoining host anorthosite. Here, the megacrysts are interpreted to have been streaming away from incompletely crystallized residual leucogabbroic liquid. Both megacrysts and pockets of residual liquid
were nucleating at about the same time. A small, but significant H2o
content and a low ilmenite content appear to favor the formation of biotite in the leuconorite, whereas the reverse, dry, oxide-richer composition favors the growth of garnet in the host anorthosite. Biotite leuconorite rafts are a characteristic feature of the Marcy anorthosite and
occur throughout the 3000'(915 m) of exposed section of the core of the
massif.
Another minor, although conspicuous, outcrop feature of the Marcy
anorthosi te is the occurrence of arcuate "ridge veins" rich in garnet.
In the Marcy massif, all other lithologic types of dike rocks are consistently less resistant to erosion than the anorthosite and are connnonly
the loci for waterfalls in anorthosite terrain. The "ridge veins",
alone, are more resistant to erosion due to the presence of abundant
quartz and garnet. The veins contain garnet, quartz, magnetite, plagioclase, and smaller amounts of augite, hypersthene, and potassium feldspar.
The source of these very late stage solutions is not apparent but may be
anorthositic.
Retracing our steps about 0.1 miles(0.16 km) brings us to the junction
of the Short and Long Trails, where, time permitting, we will detour on
a short bushwhack to view a part of the west sunnnit where two narrow
amygdular olivine-bearing camptonite dikes, samples DMJ-1 and 2 at locality
I-Jo-6(Fig. 9) form steep sided erosional slots in the more resistant
anorthosite host rock. The camptonites(Jaffe, 1953 and Fig. 10) carry
phenocrysts of fresh olivine, zoned titanaugites with good hourglass
structure, kaersutite or titanian hornblende, and spherical calcite+sodic
plagioclase filled amygdules, all lying in an andesine, An 30_ 35 , groundmass. These fine grained lamprophyres are distinguished from aiabases
only by the presence of vesicles derived from the weathering of calcite-
w
0
Table 3.
Modes of anorthositic rocks from Mt. Jo, Marcy Massif, N.E. Adirondacks.
Biotite leuconorite
Andesine anorthosite
I-Jo-6B
I-Jo-8
I-Jo-6a
-
-
56.1
43.2
I-Jo-9
Norite Pegmatite
I-Jo-2
Plagioclase
Megacryst
Matrix
57.
80.0
75.6
36.9
50.5
15.6
4.6
1.0
3.0
40.
Augite
1.0
17. 4
0.8
1.2
2.
Biotite
2.6
0.2
0
0
1.
Ilmenite
0.4
2.0
0.8
1.0
Magnetite
-
4.4
1.1
0
+
+
-+
Hypersthene
+
Garnet
0
0
Apatite
0.4
0.2
100.0
100.0
Total
100.0
100.0
100.
46.5
47.5
51.5
34.
Plagioclase
An, Megacryst
An, Matrix
48.5
46.5
Fs, Hypersthene
39.5
44.
45.
Fs, Augite
2 7.
33.
31
Color Index
20
24
46.5
7
6
43
31
t
11._
h
Pla:,:inda~· la1h~
and 1·aki1<· in an :1111y:.:dult-.
1l'lat1l' jH•lari;wd lii:hl X 150. •
TABLE
2.
MODES• OF THREE CAKPTONITE DIKES FROM MOUNT
ESSEX COUNTY,
Olivinet
Augite+pigeonite
Brown hornblende
Biotite
Magnetite
Plagioclase (An::_,•)
Calcite
Apatite
Jo,
N. Y.
DMJ-1
DMJ-2
DMJ-3
12.5
32.3
16.8
2.0
10.2
24.0
2.2
14.2
16.0
32.5
3. t
14.2
18.0
2.0
10.8
28.0
23.2
2.0
8.4
25.8
1.8
Trace
Trace
Trace
100.0
100.0
- - - - - - - ----
100.0
•All percentages by volume.
t Includes some secondary serpentine and talc.
Figure 10. Amygdular and hourglass textures, and modes of camptonite dikes from
Mt. Jo. Jaffe, 1953. Mt. Marcy quadrangle.
32
filled amygdules, and by abundant ferromagnesian mineral phenocrysts,
both lacking in diabases. The undeformed spherical amygdules suggest
that these dikes are geologically young and presumably correlative
with radiometrically dated Cretaceous lamprophyres that intrude Paleozoic strata to the east. The dikes are oriented N31W90 and transect
the strong N78W60-90 foliation in the andesine anorthosite.
Returning to the trail junction and descending the Long Trail, we
meet a third camptonite dike, DMJ-3, at locality I-Jo-5(Fig. 9) in the
trail at elevation 2700'(823 m). Its silica-undersaturated and alkalic
affinity, indicated by the presence of normative nepheline+orthoclase
(Jaffe, 1953) which are absent modally, results from the presence of
the alkalic, low-silica amphibole, kaersutite. We reach sample location
I-Jo-2 elevation 2405'(753 m) after a 0.3 mile(0.46 km) descent. Here,
pockets of undeformed biotite-bearing norite pegmatite are emplaced in
fractured garnetiferous andesine anorthosite. The norite pegmatites
consist of magnesian hypersthene, Fs34, and labradorite, An 51 .s, both
more magnesian and calcian, respectively, than those found in the enclosing host anorthosite, which carries andesine, An47, and hypersthene,
Fs45. The pegmatitic, magnesian hypersthene carries inclusions of sphene,
lamellae of titanhematite and plagioclase, and shows strong chemical and
mineralogical affinities with the giant Al-rich orthopyroxene megacrysts
of anorthosites (Emslie, 1975 and Jaffe, et al, 1983). Most orthopyroxene megacrysts in the Mt. Marcy quadrangle carry calcic plagioclase
exsolution lamellae, whereas others have exsolved pyrope-rich garnet.
Notes:
).....
Nye
Ma.re~ AnoYthosite
CoY"e Fo.ci.es
A. ho,. t1, 0 si. t: c
G '?etss
0
I
0
1
1
I
:JO
cm
Mt. Va.n HoeVeVlbeY"g
West ele VCl.t ~o"'
Figure 11. View west to Nye Mt. from ledge below summit of Mt. Van Hoevenberg, showing oxiderich melagabbro cumulate layers recumbently folded in Van Hoevenberg gabbroic anorthosite
augen gneiss. A schistosity cuts the folds and dips in the opposite direction.
Mt. Marcy quadrangle.
w
w
34
Stop 2.
THE VAN HOEVENBERG GNEISS-GABBROIC ANORTHOSITE AUGEN GNEISS
WITH LAYERS OF OXIDE-RICH MELAGABBRO-ROOF FACIES
Drive five miles (8 km) north on Heart Lake Road, 3.5 miles (5.6 km)
east on N.Y. Route 73, and one mile(l.6 km) south on entrance road to
the Mt. Van Hoevenberg parking lot at the base of the site of the bob
sled and luge runs of the 1932 and 1976 Olympic Games. We will be ferried
by bus to the top of the bob sled run at 2500'(762 m) and will climb
by foot trail to the prominent 2860'(842 m) flat summit of Mt. Van
Hoevenberg. A series of flat ledges look to the southwest across the
South Meadow lowlands to the prominent anorthosite cones of Mt. Colden,
4714'(1437 m) and Algonquin Peak, 5114'(1559 m) in the core of the Marcy
massif.
The country rock, an excellent geologic mapping marker unit, is the
hornblende-garnet gabbroic anorthosite augen gneiss. It is a highly
deformed gabbroic-noritic anorthosite containing cumulate layers of
inverted-pigeonite pyroxenite diluted by plagioclase melt to produce
a melagabbroic composition(Table 4 and Fig. 11). It has also been contaminated by the incorporation of small amounts of granitic melt. The
thin layers of the melagabbro, and the constancy of composition of plagioclase and pyroxenes of the anorthosite gneiss and the mafic layers,
suggest that the latter are cumulate layers rather than sill-like injections. Gneissic foliation is enhanced by the folded mafic layers and
by crenulated streaks and thin layers of hornblende+garnet+augite
wrapped and bent around abraded augen of calcic andesine. Sporadic
xenoliths of granulated andesine anorthosite and supracrustal rocks orient
as flattened plates in the well-defined plane of foliation, which is
oriented Nl1Wl5NE, a mineral lineation trends N20El5(Fig. 12). A later
schistosity transects the foliation at a low angle(Fig. 11). During
the Grenville orogeny, isoclinal recumbent folding was succeeded by
thrust faulting, during which time andesine anorthosite of the core of
the Marcy massif was thrust from the west and southwest to the north
and northeast over the gabbroic anorthosite, quartz ferromangerite, and
supracrustal gneisses of the roof facies of the massif. A major low
angle fault, the Basin Thrust, exposed on Basin Mt. to the south, separates the anorthosite core from the gneissic rocks. Where exposed, the
Basin Thrust dips gently to the west and strikes in a N20W direction.
Thus, it extends from Basin Mt. along Klondike Brook to South M~adow
between Jo and Van Hoevenberg summits. Below our vantage point it is
transected by the Cascade Lakes Fault, one of the major N40E steeply
dipping fracture zones that dominate the topography of the Northeastern
Adirondacks. The Cascade Lakes Fault has down-dropped the northern
block (Pitchoff block) containing the quartz ferromonzonite gneisses
of Pitchoff Mt., the gabbroic anorthosite gneiss of Mt. Van Hoevenberg,
and the andesine anorthosite of Mt. Jo in the core of the massif, from
the adjoining Marcy Block to the Southeast.
We will return to the parking lot by foot enjoying the opportunity
to view the construction and curves of the bob sled run in detail.
Schistosity
Figure 12. Detail of folded oxide-rich melagabbro cumulate layer in Van
Hoevenberg gneiss; foliation Nl2Wl5E; mineral lineation N20El5 is
parallel to the fold axis, and is cut by west-dipping schistosity.
Mt. Marcy quadrangle.
36
.Table 4.
Modes of the gabbroic anorthosite augen gneiss(Van Hoevenberg
Gneiss) and ma.fie layers from Mt. Van Hoevenberg summit.
VH-1
VH-lm
Quartz
0.1
0.1
Microperthite
7.0
+
Megacryst
15.6
26.4
Matrix
58.4
+
Plagioclase
Hypersthene
1.8
15.0(Inverted pigeonite)
Augite
8.8
28.0
Hornblende
2.2
6.4
Garnet
2.8
10.7
Ilmenite
2.4
7.3
Magnetite
0.6
6.0
Apatite
0.3
0.1
100.0
100.0
An, Megacryst
45.5
43.5
An, Matrix
43.5
43.5
Fs, Hyp.
57
52.
Fs, Aug.
44.
44.
Color Index
19.
74.
Total
37
Stop 3.
GRENVILLE MARBLE-SYENITE-ANORTHOSITE SECTION SOUTH OF KEENE
Drive five miles(B km) northeast on Route 73 to the first right
turn after a Gulf gasoline station and almost into Keene village
center. Turn right(south) on the westernmost of two small roads that
parallel both sides of the East Branch of the Ausable River. Drive
about 0.75(1.2 km) miles south on this western side of Hulls Falls
Road and park judiciously along the edge of this little travelled road.
Descend about 25'(7.6 m) to the bank of the river watching out to avoid
standing or sitting in Rhus toxicodendron which commonly grows in
Grenville marble terrain. A fine river outcrop of folded Grenville
marble consists of calcite(white), diopside(green), fluopargasite(black),
and minor glistening flakes of phlogopite(brown) along with less abundant graphite. Pink quartz leucosyenite and black-streaked gray-white
gabbroic anorthosite gneiss have been dragged into highly contorted syntectonic folds enhanced by the plasticity of the marble and the probable
molten state of the quartz syenite and gabbroic anorthosite(Fig. 13).
Occasional tongues of gabbroic anorthosite cross-cut the syenitic rocks.
A major vertical fracture zone, the Keene Fault Zone runs parallel to
the river in a N-S direction, and is well exposed about one-half mile
(0.8 km) south in a granulated anorthosite outcrop. The Keene Fault has
dragged the preexisting, gently north dipping, isoclinally folded strata
into fairly steeply plunging folds at this locality. A late, brittle
stage of movement on the same fault has granulated and retrograded all
of the brittle rock types. Feldspar in syenite is sericitized, intermediate plagioclase in gabbroic anorthosite has been albitized and veined
by calcite, grossular-diopside calc-silicate rocks have been prehnitized
and chloritized, but marble merely goes along for the glide.
At the northernmost end of the outcrop, the diopsidic marble and the
quartz syenite are transected by a 4'(1.2 m) wide N80W90 trending lamprophyre dike(Fig. 13) which displays good chilled margins. It is a classic
lamprophyre: a dark, dense, porphyritic dike rock in which the ferromagnesian minerals occur in two generations and in which only the dark minerals
form the phenocrysts. It consists of 1-5 mm diameter phenocrysts of
partially serpentinized magnesian olivine, and zoned clinopyroxene with
augite cores and titanaugite rims, which display spectacular zoning, intense anomalous interference colors and dispersion, and hourglass structure. The groundmass contains a second generation of microphenocrysts
of titanaugite, kaersutite, titanian biotite and abundant very thin needles of apatite in a quasi-isotropic base that has too high an index of refraction to be analcime or leucite; it has a mean index of refraction
= 1.525 and is either untwinned anorthoclase or a zeolite. The dike
may be classified as either a camptonite or a monchiquite, but exactly
conforms to neither.
Notes:
w
00
o
<~
N<
...
'9
~
...
~
0
10'
•S'
...
T
+
.
......
.. o •
~
.. .. Cb . ...
+~.,.
......
...
~:
I
+
~
s'
.· . ~1'
~
Coneeo. led
.,.OI ..
f
't
•
"
,. ,
,.....
..
~
..
.,.
•
f
•
/
\
. /'/<
~
4..
• •'ti
.
I
. ., ..,.! / /
..
...
t
~
... ~.,, ; I .,,
C•
,.
....
'\
.....,
,~,.,
/
\
, ,\,, \.,,
... , ;
'.,,
--
-
"'" , ..,,....
c,o"'c. "'"'\eP
/
't'
ft.er Kenil:', t '11 1 P·' 't
Figure 13. Contorted folding in diopside-calcite-pargasite-calcite marble, quartz leucosyenite
gneiss, and anorthosite gneiss in the West Branch of the Ausable River south of Keene, N.Y.
A camptonite dike cuts the marble-gneiss section. After Kemp, 1921. Mt. Marcy quadrangle.
39
Stop 4.
THE BEEDE HILL GRENVILLE-ANORTHOSITE HYBRID GNEISS, BH-SB
Drive one-half mile(0.8 km) south, cross an old iron bridge, turn
right,(south) on Hull's Falls road, and drive one mile(l.6 km) to the
new bridge at Hull's Falls. Park on the north side before crossing
bridge, and walk back about one-quarter mile(0.4 km) to the western edge
of Beede Hill, a low, thickly wooded promontory between Hull's Falls
road and Route 73 to the east. Beede Hill consists mostly of Grenville
lithologies: garnet-quartz syenite gneiss, pyroxene-plagioclase gneiss,
hornblende granite gneiss, amphibolite, garnetiferous alaskite gneiss,
and calc-silicate granulites. A thin but conspicuous anorthosite sill
outcrops at about the 1300'(396 m) elevation near the top of the 1470'
(448 m) hilltop. Outcrops at the summit are of tightly folded amphibolite and granitic-syenitic-monzonitic gneisses carrying sporadic 2-4cm
(0.8-1.6") euhedral to subhedral blue-gray megacrysts or xenocrysts
of calcic andesine of anorthositic parentage. Matrix plagioclase in
the felsic gneisses is sodic andesine, An 30 , and the calcic andesine
xenocrysts, An _50 , are markedly out of chemical equilibrium with the
46
host rock plagioclase. In addition to plagioclase, hypersthene, augite,
hornblende, and microperthite are present along with minor amounts of
opaque oxides. Miller(l918) coined the name, "Keene Gneiss" for such
rocks, and deWaard(l969) classed them as jotunite, a proposed name for
metamorphosed hypersthene-bearing monzodioritic rocks of a supposed
"charnockitic series". We suggest that these gneisses result from
the mixing of anatectic felsic melts with ·partially crystallized
anorthosite melt resulting in the incorporation of calcic andesine as
xenocrysts in felsic rocks. It is a popular misconception of Adirondack geology that such xenocrysts are common in quartz syenite and
granitic gneisses, which instead commonly carry blue-gray iridescent
microperthite phenocrysts or porphyroblasts, but not calcic plagioclase.
The xenocrysts of calcic andesine occur predominantly in monzonitic to
monzodioritic gneisses near their contacts with anorthosite.
Time limitation will not allow us to climb Beede Hill but instead
we will descend to the east bank of the Ausable River where a superb
example of a plagioclase-xenocrystic hybrid augen gneiss is exposed.
Exercise great caution in descending to the river as the slope is very
steep, slippery, and treacherous. Take care not to tumble loose rock
on your neighbor!
The prominent river bank outcrop shows Schiller-iridescent, bluegray 2-5 cm(0.8-2") xenocrysts of calcic andesine, An4 8 • 5 , rimmed by
white mesoperthite in a well-foliated gneiss(Fig. 14). The matrix of
the gneiss contains plagioclase, An35, hypersthene, Fe60' augite, Fe44.
garnet, ilmenite, hornblende, and minor amounts of microperthite. The
host rock is a two-pyroxene-plagioclase gneiss with a few thin granitic
stringers. Careful examination should reveal a few thin concordant
tongues of anorthosite which locally transect the gneissic foliation;
presumably, these derive from the anorthosite sill on Beede Hill. If
the parent rock of the sodic plagioclase gneiss had invaded an anorthosi te, it would contain xenoliths and not single xenocrysts of the
higher-melting-temperature anorthosite. Conversely, if the partially
crystallized anorthosite melt induced anatectic melting of more alkali-
~
0
~
._,...
I
0
·
10
l
.2-0C.\'\"\
BH-5 B
Figure 14. Blue calcic andesine megacrysts in Grenville-anorthosite hybrid gneiss in the West
Branch of the Ausable River south of Keene, N.Y., at the west edge of Beede Hill.
Mt. Marcy quadrangle.
41
rich layers, the latter would inherit and preserve the higher-meltingtemperature calcic andesine as single crystal xenocrysts. The resulting rock is a Grenville-anorthosite hybrid gneiss.
Here the Grenville gneiss section strikes N85E, and dips gently
(15-30°) to the north. A very short distance north, the hybrid gneiss
grades to a biotite-hypersthene-augite-plagioclase gneiss devoid of
both the plagioclase xenocrysts and garnet. Several pyroxenite layers
containing hornblende, hypersthene, and augite are intercalated, and
further north and up-section, calc-silicate granulites made up of grossular, diopside and quartz are abundant. Prehnite is common in the latter
and may be retrograde. Contrast the gentle, north-dipping attitude of
the layers here with the more contorted steep attitudes of the marblesyenite-anorthosite section seen at the previous stop, 0.75 miles
(1.2 km) to the north.
Notes:
42
Stop 5.
HULL'S FALLS- SYENITE-ANORTHOSITE HYBRID GNEISS, TRANSITION
ROCK or CHARNOCKITE-MANGERITE-JOTUNITE?
We will stop for lunch at Hull's Falls and make a brief examination
of the outcrops. The Hull's Falls section represents a more advanced
stage of anatexis of the felsic supracrustal sequence of the previous
Stop 4 at Beede Hill river bank. Here at Hull's Falls the prevailing
E-W(N75E - N75W) strike and moderate northerly dip of the gneissic
strata conform closely to those seen at Beede Hill but here the dips
are steeper, from about 30N to about 30-50N. The Hull's Falls section
consists of a markedly layered sequence of: hornblende-hyperstheneaugite-plagioclase, An28-35'(garnet) gneiss carrying sporadic xenocrysts of calcic andesine(jotunite of deWaard, 1969); orthopyroxenequartz-bearing monzonitic gneiss(mangerite to farsundite of deWaard,
1969); and eulite-bearing granitic gneiss(charnockite of deWaard, 1969).
Cushing(l905) and Davis(l971) classed gneisses of this type as transition rock, placing them in the quartz syenitic series of a TupperSaranac sheet. A marked increase in both the volume occupied by granitic layers and in the prevalence of plagioclase xenocrysts, with respect
to Beede Hill outcrops, is attributed to the proximity of the large reservoir of gabbroic anorthosite magma that crystallized here, and which
dominates the eastern part of this map area. Iron-rich orthopyroxene
compositions in these gneisses range from 100Fe/(Fe+Mg)=69-80, matrix
plagioclase, An2 8_ 35 , calcic andesine xenocrysts, An47-50, augite,
lOOFe/(Fe+Mg), 60-72, the clinopyroxene carrying abundant metamorphic
exsolution lamellae of pigeonite and.hypersthene. Garnet and hornblende
abound in the more mafic plagioclase-rich layers and tend to be absent
from the granitic layers, an expression of the preference of garnet
nucleation for Al-rich loci supplied by more anorthite-rich plagioclase.
Notes:
43
Stop 6.
THE BAXTER FOLD IN NORITIC ANORTHOSITE OF BEEDE LEDGE
Turn east off Route 73 three-quarters of a mile north of the
village of Keene Valley at trail sign to Baxter Mt. Bear left after
the bridge and again left and uphill towards Lone Pine Cottage. Pass
private road to the left and park along the right. Return to private
road and walk uphill past gate and chain to outcrop at bend in the
road.
Here we see a noritic anorthosite that has been thrown into an
open fold whose axis is Sl2El5. On the left limb of the open fold can
be seen the tight hinge of an earlier stage recumbent fold in the noritic
anorthosite(Fig. 15), with an axis N8W22. At the right hand side of
the outcrop, steep fluted and slickensided surfaces make further interpretation of the folding difficult. It appears that the recumbently
folded anorthosite w.as dragged into an open fold by a fault most likely
related to the N-S Keene Valley fault zone.
The noritic anorthosite consists primarily of andesine and hypersthene with very little augite. Some hornblende retrogrades the hypersthene, and minor apatite and ilmenite are present. The absence of
garnet is noteworthy, and results from the low iron content of the
orthopyroxene, 100Fe/(Fe+Mg)=39. Martignole and Schrijver(l973) and
Jaffe et al(l983) have noted that garnet will be absent from anorthositic rocks when the Fe ratio is less than about 45.
r.Jiv'
((.,.c.
v-"
e,'o ~
.
I
e~
'
l"'
,...
"
NoY-th elevo..li..on
.;,...
Figure 15. Refolded recumbent fold in noritic anorthosite at Beede
Ledge, Baxter Mt. Mt. Marcy quadrangle.
44
Stop 7.
SCAPOLITE HILL AND KELLY'S LOGS
Return to Route 73 and drive 1.3 miles(2.l km) north angling right
on a dirt track beyond a Town of Keene garage, and parking in a small
picnic area on the west bank of the Ausable River.
BE CAREFUL IN CROSSING HIGHWAY 73. Traffic is rapid here on one
of the straighter sections of this highway.
Assemble at a small wooden building at the edge of the Marcy airfield. The large open meadow is sporadically used both as a trotting
track and as a small landing strip for light aircraft. Walk directly
across the meadow from the wind sock at the building into a wet, often
swampy thicket. We will traverse or thrash through about 500' of boggy
scrub growth and emerge at the talus-covered base of a steep cliff
where the normally easy going has been made difficult by a recent,
very sloppy attempt at a lumbering operation. Rubble and huge boulders
contain the simple mineral assemblage: scapolite-hedenbergite-calcite
(sphene) formed from the reaction between ferruginous siliceous carbonate rocks and anorthositic(and perhaps syenitic) magma. Some of the
huge boulders contain giant single crystals, to 25 cm x 3 m (9.6" x
9.8') of pearl gray weathering, pale blue-gray, tetragonal prisms,
often well terminated, of meionite-rich scapolite. We have named these
megacrystals, "Kelly's Logs", after our colleague, Bill Kelly of the
N.Y. State Geological Survey, who found them while working on his M.S.
thesis(Kelley, 1974). The very abundant black euhedral crystals are
hedenbergite, 100Fe/(Fe+Mg)=67-95. Many of the large crystals of
scapolite are calcite-cored. According to Kelly(l974), the scapolite
crystals occur in a 60-70 m (197-230') thick layer floored by gabbroic
anorthosite and capped by a monzonite. In detail, we find the relations
more complex as indicated in the block diagram (Fig. 16). Scapolite
crystals occur in disconnected pods and dismembered layers of scapolitehedenbergi te-andradi te-sphene-calcite skarn assemblages wrapped in a
thin layer of forsterite-diopside marble and ferrosyenite-ferromonzonite
gneiss. The calc-silicate horizon is succeeded by ferrosyenite and then
more anorthosite, then, by a thicker layer of ferrosyenite which gives
way near the hilltop to a folded pyroxene amphibolite clearly intruded
by anorthosite.
At the base of the hill, gabbroic anorthosite and a thin discontinuous
monzonitic layer are succeeded by a scapolite-hedenbergite gneiss and
the forsterite-diopside marble, 1.5 m(4.9') thick, in turn overlain in
sharp contact by a 10 m (33') thick layer of hedenbergite-scapolitecalcite-sphene rock. , the outcrop source for the giant crystals seen in
the huge boulders. Many of the giant crystals are oriented with their
long axis perpendicular to the forsterite marble contact. At an outcrop
about one-quarter mile to the north, giant scapolites are found growing
from the floor and roof of a 20' x 8' elliptical pod of calc-silicate
completely enclosed in gabbroic anorthosite, Fig. 17. The calc-silicate
pods apparently represent pockets of co 2-rich volatiles that were trapped
in the anorthosite magma. Their growth perpendicular to the floor and
roof of the elongate pods and their often well-formed pyramidal terminations are consistent with growth pattern in a vein or pod. They grew
Gl \'\O \'
,)
/
-
t.
'
' _.. ,....,,...
,,,.,......._
,,, ...
hOJOi.'t'e ' /
.
,_ 7
-
....._ / _
/
__-
~
,..,
'/
~
fer ros !I e "1 i. te
1 ioo-"'
3'-"
'
'~ /,-,,.,..,
' '
a.!!or.tho.si.te
./-.,,,,
1100'
335'111
--.•..-n•-,
/
fe..-ros:ie7>1t-e
. ~
_
/
'
,, -
-r,e rcalc 1c
a.northosl,te
/
\
• '1000 1
~
--~-- -
.
'- -
~
-'
'
.:;:::--- -
/
,,.,- .........
'-
,...-
"'
/
\
-
"~ / '
~,...._,
/
"-
,
_..
,...' -
1.2.KWI
Sca.rol~te
\,Jest
H~ll
e(eva.tio-,.,
Figure 16. Schematic block diagram of west-dipping folded scapolite-hedenbergitecalcite skarn, ferrosyenite, and percalcic anorthosite, of Scapolite Hill,
Mt. Marcy quadrangle.
~
VI
~
°'
-""
\"~&·--\~~/
/
\
"'-
""-
\_
~
·---) >~- ~
\ / '~
'
~
-
~Ji
"-.
Si L
\_ /
//
----wC"
" >/ "
---~
..--~
--
I
'-...
/
""'
/
......_
......_
.....
/
\
/
l
\.
\_...
/
'\..
/
c""
"
~
"""
1~
r"
I
I
I
I
I
•
o(
,.
""--
~
(
"
""'
'-.,
l
/
/
""'
0
f,
Figure 17. Detail of 20' x 8' (6.1 x 2.4m) skarn pod, showing well-terminated crystals
of scapolite and hedenbergite growing perpendicular to margins of pod. Calcite is
leached from center of pod, which lies in foliated percalcic anorthosite. Scapolite
Hill, Mt. Marcy quadrangle.
47
toward the center of the pod.by progressive replacement of calcite which
has now been leached out leaving the voids which have a v:ug-like appearance. The typical assemblages found in the calc-silicates evidently
grew by the following reactions, separately and in concert:
+ Feco 3 + Fe 2o + 5Sio 2 = CaFeSi 2o + Ca Fe si o + 5C0
3
6
3 2 3 12
2
4Cc + Sid + Hm
+ 5Qz
=
Hed
+
Andradite + co • . • (1)
2
Caco + Ti0 2 + Si0 2 = CaTiSio + co 2
5
3
Cc + Anat + Qz = Sphene + co 2 ••••••••••••••••.••••••••••••• (2)
4Caco
3
3 CaAl Si o
2 2 8
3 An
+ Caco 3
+ Cc
ca Al Si o (co )
4 6 6 24
3
=
Meioni te (Me) ............................ . (3)
+ 0.33 NaCl
=
+
=
Hl
Na1 • 33A1Si 30 8c10 • 33
Mariali te (Ma) ....................... . (4)
Two wet chemical analyses of scapolites from these outcrops show
and Me (Table 5) after Kelly, 1974.
limited ratios of Me
64
66
Combining the four reactions (1 through 4), above, we may write:
6 Caco
3
+ FeC0 + Fe o + Ti02 + 6Si02
3
2 3
6 Cc + Sid
+ 3 CaA1 Si o
2 2 8
+ 3 An
+ Hm
+ Anat + 6 Qz frqm the carbonate rock
+ NaA1Si o + NaCl
3 8
+
Ab
+ Hl
from the anorthosite magma
CaFeSi 2o 6 + Ca Fe 2 si o 12 + CaTiSi0 + Na • ca A1 si o (co )cl • +6C0
3
3
1 33 4 7 9 32
5
2
0 33
3
Hed
+ Andradite
+ Sphene +
Scapolite
This is the most common assemblage on Scapolite Hill.
Notes:
(5)
48
Table 5.
Wet chemical analysis of scapolite, KV-NWl and KV-lX
Analyst:
Tadashi Asari, in-Kelly(l974)
Number of ions
per 26 (O,F,Cl)
Weight per cent
47.47
46.98
Si
0.17
0.04
Ti
27.50
27.55
0.37
0.50
15.81
16.09
FeO
0.28
MgO
0.14
Si0
2
Ti02
Al o
2 3
Fe o
2 3
CaO
MnO
Al
Fe+3
7.1491
0.019
12.09
4.883
7.024
0.004
_J .
4.857
0.042
'
0.34
Ca
Fe+2
2.5511
0.056
0.035
0.19
Mg
0.032
0.042
0.01
Mn
0.271
~~~~~l
0.649
0.004
0.239
3.37
3.37
Na
~~~~~
K20
H o(-)
2
H o(+)
2
F
1. 72
1.42
K
0.331
0.49
0.55
0.58
0.65
C0 2
H
0.042
J
Na 20
(+)
2.578
0.01
F
0.73
1.17
c
0.150
Cl
0.82
0.73
Cl
S0 3
0.36
0.32
s
0.209
0.041
P205
TOTAL
0.01
0.03
99.81
99.92
O=Cl
0.18
0.16
TOTAL
99.63
99.76
I
11. 941
_J
I
I
3. 934
0.983
.J
3.910
0.977
0.185
0.036
J
I
1.113
_J
49
Stop 8.
THE PITCHOFF GNEISS - FERROSYENITE FACIES, P0-2
Turn right(north) on Route 73, and go through Keene Village, where
Route 73 turns west. Follow it about 4.5 miles(7.2 km) to the foot
of Lower Cascade Lake. Park at the lakeside in the second parking area
on the left, just past a sign FALLEN ROCK 1 1/2 MILE. Be very careful
cutting across traffic: this is a busy high-speed highway. Recross the
road on foot, again with great caution. Walk back toward the FALLEN ROCK
sign, to a rough trail up the talus just short of the sign. The talus
and cliff are both steep and full of loose rocks: be considerate of those
below and behind.
The prominent, southeast-facing cliff we are climbing to is a ferrosyenite gneiss that crystallized from a melt prior to its metamorphism.
It is one of a group of quartz-poor, alkali-feldspar- and ferroan-pyroxenerich igneous rocks that acquired their gneissic fabric during an episode
of isoclinal and recumbent folding associated with the Grenville orogeny
at about 1100 M.Y. The persistent proximity and intimate intercalation
of these gneisses with "Grenville" supracrustal rocks suggests that they
may have initially been iron-rich felsic volcanics, or perhaps sills,
interlayered with siliceous dolomites, calcareous quartzites, marls and
basaltic flows comprising a Proterozoic series of rocks deposited about
1350 M.Y. This Grenville age is estimated from Ashwal's(l983) recent
sml47_Ndl43 date of 1288 M.Y. believed to represent the age of crystallization of the Marcy anorthosite massif-core facies. Following the
model of McLelland and Isachsen(l980) for the southern Adirondacks, we
suggest that, in the High Peaks Region of the northeastern Adirondacks,
a "typical" Grenville supracrustal sequence correlative with rocks of
the Central Metasedimentary Belt(Wynne-Edwards, 1972) was buried in a
plate-tectonic event or events to a depth of about 70 km(42 mi) that of
a doubly thickened crust. Following Emslie(l977), we envisage the birth
of an anorthositic magma from the fractionation of copious amounts of
orthopyroxene from an already-fractionated Al-rich gabbroic magma. Under
these deep-seated conditions, the high pressures and temperatures plus
the availability of Grenville-strata-derived co -rich fluids initiated
2
the formation of potassium- and iron-rich, relatively quartz-poor, melts
of syenitic to monzonitic composition(Wendlandt, 1981). Ascent, intrusion,
and emplacement of the syenitic melt at levels on the order of 25-35 km
(15-21 mi) and temperatures of 800-900° induced deep contact metamorphism
of appropriate Grenville rock types. Here, at the easternmost part ot
the P0-2 outcrop, designated P0-2Gv(Fig. 18), a calc-silicate sequence
infolded with the ferrosyenite contains the assemblage: wollastonitediopside-grossular-quartz. Because anorthosite is absent, the contactmetamorphic origin of the wollastonite must be attributed to the intrusion of syenitic melt. Further, because the ferrosyenite contains relict
inverted pigeonite, which now consists of host orthoferrosilite(lOOFe/
(Fe+Mg)=85-92), the melt must have crystallized at temperatures of 8509000(Lindsley, 1983 and Ollila and Jaffe, in preparation). Alternatively,
shallow emplacement with crystallization of fayalite and quartz, later
deeply buried and converted to orthoferrosilite and quartz, is conceivable, yet unlikely, because no relict olivine, whatsoever, has been observed by Jaffe or by Ollila in quartz-bearing syenitic rocks of the Mt.
Marcy and the Santanoni quadrangles. Fayalite (Fa95 ) plus quartz, but
so
Figure 18. P0-2gv. Folded amphibolite, marble, and wollastonitebearing calc-silicate rock in ferrosyenite gneiss. Pitchoff
Mt. cliff, above north end of Lower Cascade Lake. Mt. Marcy
quadrangle.
P0-2 Gv and P0-2 f
P0-2 Gv
Covered
-
-- -
Coarse marble
~
Covered
51
Modes of rocks at Stop 8
.TABLE 6.
Ferrosyenite gneiss
P0-2
host rock
, locality P0-2
Shonkinite
P0-2m inclusion
Q
1.5
2.0
Mp
84.5
84.0
50.0
Plag
3.6
4.0
13.0
Aug
4.1
4.0
12.7
Hy
2.4
2.0
16.6
Hb
1.8
2.0
Ore
1.0
Gt
Amphibolite
Inclusion
Aplite
P0-2d dike
23.0
60.0
30.0
16.0
4.1
58.0
0.9
1.0
2.6
+
0.1
0.8
1.0
+
Ap
0.3
+
1.0
Zr
+
+
+
+
Bio
100.0
100.0
100.0
21
21
Fs(opx) 83-89
75
CI
40
An
15
o.o
2.0
10.0
100.0
100.0
88
1
Mineral assemblages infolded in ferrosyenite gneiss
host rock at locality P0-2Gv.
Amphibolite: hornblende and altered plagioclase.
Cale-silicate: hedenbergite-plagioclase-quartz
hedenbergite-plagioclase-quartz-scapolite-sphene
diopside-grossular-quartz-sphene
wollastonite-diopside-grossular-quartz
wollastonite-calcite-prehnite
diopside-calcite
tr
V1
N
P0-2
North elevation
J
5,•
0
·1
0
I
'
I
(
I
l.·
1m
.........
trace of f oliiltio
N45E 30NW
Grain size of host 2.0 mm
Grain size of inclusion 0.5 mm
Figure 19. Xenolith of folded shonkinite layer P0-2m and amphibolite remnant in quartz
ferrosyenite gneiss P0-2. Aplite dike cuts syenite gneiss. Pitchoff Mt. cliff
above north end of Lower Cascade Lake, Mt. Marcy quadrangle.
.......
~
53
with orthopyroxene absent, has been described from quartz-syenit:i;c rocks
in the Cranberry Lake quadrangle to the west(Buddington and Leonard,
1962, Jaffe et al. 1978) and in the Ausable Forks quadrangle to the
northeast(Kemp and Alling, 1925 and Jaffe et al, 1978). A deep emplacement with high pressure crystallization is consistent with field observations and experimental data for all of these rocks.
At the P0-2 outcrop, we will split into several smaller groups: the
footing can be a bit tricky. Remember not to step back for a better
look at the outcrops. The first or westernmost cliff consists of
strongly foliated ferrosyenite gneiss, N45E30W, with a large inclusion
of shonkinite granulite(Fig. 19, Table 6). The foliation continues
through the inclusion. The southwest end of the inclusion is sharply cut
off by the gneiss but the northeast end fingers out. The inclusion is
cut by a discordant vertical tongue of gneiss which becomes a subhorizontal pegmatite vein. At the northeast end of this cliff, the ferrosyenite gneiss is cut by an unfoliated aplite dike. Small amphibolite
inclusions can be seen in the gneiss.
We will proceed cautiously about 30d(91.5 m) along the base of the
cliff to the northeast, across a stream and a gully. Here we see several
larger folded amphibolite inclusions in the ferrosyenite gneiss(Fig. 18).
The axial planes of these folds are approximately parallel to the pervasive foliation. We will now crawl a few feet up the gully: in its east
wall are exposed marbles and calc-silicates intimately infolded in the
ferrosyenite gneiss. Wollastonite occurs in these calc-silicate beds
(Fig. 18). If we make allowance for the plasticity of the marble, these
folds are also approximately parallel to the pervasive foliation. There
is a cave in the marble a little higher up this gully. On the opposite
side of Lower Cascade Lake, in the anorthosite, another cave can be
found a few hundred feet higher up. Caves are common in New York State,
but these two must be among the few in Precambrian rocks.
Notes:
54
Stop 9.
HEAVEN HILL AND ANORTHOSITE SILLS
Proceed west 7.6 miles(l2 km) on Rte. 73. Bear left on Old Military
Road following the signs for Saranac Lake. Proceed approximately 0.8
miles(l.26 km) and take a left just before reaching a hospital. Drive
2.3 miles (3.7 km) on the paved road and stop where the pavement ends.
Stop 9A is about 0.5 miles (.8 km) further down the dirt road. The
outcrop is north of the road on a steep hillside. Stop 9B is just beyond the northeast corner of the large field on the north side of the
road where the pavement ends. These outcrops are on private property:
obtain permission at Heaven Hill farm.
Stop 9A is an outcrop first described by Buddington(l953, p.74) as
"intrusive lenses of Whiteface gabbroic anorthosite between the Grenville and quartz syenite". Stop 9B is northeast of 9A at the base of
Heaven Hill. Here, marble, calc-silicates, amphibolite, and hornblende
granite gneiss dip to the north and are the first rocks exposed going
northward from stop 9A. These rocks are part of the Newman syncline
described by Buddington. This northeast-trending doubly-plunging synclinal structure occupies an area overlapping the intersection of four
15' quadrangles: Saranac, Lake Placid, Santanoni, 'and Mt. Marcy. Section
A-A', Fig. 20, is drawn from 9A through 9B and roughly parallels the
trend of the Newman syncline.
9A. The southwest side of this hill consists of interlayered gabbroic
anorthosite gneiss and garnetiferous granitic gneiss. Rocks equivalent
to the Pitchoff Gneiss-ferrosyenite facies, outcrop inunediately to the
south of this hill and dip underneath it. The anorthositic gneiss
weathers white and is white on a broken surface. It consists of plagioclase, An 35 ,5, and lenses and layers of augite, MG*=68, olive-green to
brown hornblende, and sphene. Hornblende, augite, and sphene occur together in elongate polygonal aggregates. Sphene is relatively coarsegrained and quite abundant in this rock. Sphene is unusual in anorthositic rocks and probably results from contamination of the anorthosite
sills during intrusion.
Anorthosite is connnonly surrounded by a rock that is rusty brown
on a broken surface. This rock consists of plagioclase, An 29 , layers
and lenses of augite, MG 50, and hornblende, and layers and lenses of
microperthite. Locally the rock contains blue plagioclase augen, An45•
The intermediate pyroxene compositions and the blue plagioclase augen
suggest that the rock is a hybrid between anorthosite and syenite.
Both of these rock types occur within a medium grained garnetiferous
granitic gneiss. The gneiss weathers white to tan and contains 2 to 5
nnn. garnets. It is heterogeneous, containing both hornblende- and
plagioclase-rich layers and quartz-rich layers. Most of the rock consists of anhedral quartz and microperthite with minor amounts of sphene,
apatite, zircon, garnet and opaques.
An approximately lm(3.3') thick body of pink weathering hornblende
granite gneiss cuts anorthositic gneiss on this hillside. A similar
*MG=lOO x Mg/(Mg+Fe)
55
Figure 20. Section showing position of anorthosite sills near Heaven
Hill, in the northeast corner of the Santanoni quadrangle.
SW
HEAVEN HILL
A
Fci..~1c; ~ NE:ISS,
A,.,,t1111'> 0 Jo I T1'
NE
A'
2000
~AR.BLE.
-
1
-- -- ---t"'-::.
Figure 21.
T - XCO
750° maximum
I000
1
diagram for the system sa-en-q-H
s~ability
0
of ph-q-V assemblage.
v~s.._
2
o-co 2
showing
1
56
body has been observed in syenitic rocks further to the southwest in
the Santanoni quadrangle. The rock is medium grained and consists of
approximately 70% microperthite, 25% quartz, 3% green hornblende and
2% reddish-brown biotite. Accessory minerals include plagioclase,
opaques and zircon. This rock is petrographically very similar to the
granitic gneiss at stop 9B and is thought to be equivalent to the
reddish granite described by Buddington (1953, p. 73-74) in the Saranac
quadrangle.
It is uncertain whether the repetition of layers in this outcrop
is caused by isoclinal folding or whether there are numerous sills
of anorthosite. No folds have been observed on this hillside but isoclinally folded pyroxene scapolite gneiss and marble near the base of
Heaven Hill and the folding seen at stops 8 and 12 suggest that repetition by folding is a likely possibility. A mineral lineation in biotite gneiss at the base of Heaven Hill trends northwesterly. If fold
hinges are parallel to this lineation it could explain why hinges are
difficult to find on this hillside. A foliation that cuts across
mafic layers at a low angle has been observed at a few localities on
this hillside.
9B. Mineral assemblages and optically determined mineral compositions from rocks at Heaven Hill are listed in table 7. The stratigraphic
section(Fig. 20) from base to top consists of 1) coarse marble and
scapolite bearing calc-silicate gneisses, 2) pyroxene-bearing amphibolites, biotite gneiss and calcareous quartzites, 3) hornblende granite
gneiss, and 4) interlayered felsic gneisses, amphibolites and calcareous
quartzites.
The contact between unit 1 and rocks to the southwest is not exposed. The maximum possible thickness for this unit is approximately
250 m(820'). Kemp(l921) reported wollastonite from marbles contiguous
with this unit in the northwestern corner of the Marcy quadrangle. Samples HV-1, 4-4, 4-16 and presumably LP-200 are from unit l(Fig. 20 and
Table 7). Pyroxene scapolite gneiss and marble are tightly folded in
an outcrop southwest of the base of the hill in the swampy area.
Samples 4-16B and C were collected from unit 2 and are samples of
pyroxene bearing amphibolite and biotite gneiss respectively. Sample
4-16C is a granoblastic gneiss with an average grain size of 0.3 mm.
Foliation is defined by aligned biotite. The most interesting textural
feature of this rock is that coarse(up to 7mm.) poikiloblastic orthopyroxene encloses augite, plagioclase and biotite. The biotite within
the orthopyroxene is aligned and the orthopyroxene has clearly grown
across the preexisting foliation. Biotite gneiss is overlain by thin
quartzites and then hornblende granite gneiss(unit 3), Unit 2 is no more
than 10 m(33') thick.
Hornblende granitic gneiss is similar petrographically to the hornblende granite gneiss at stop 9A. It is a medium grained gneiss consisting of 70% microperthite, 25% quartz, and 5% green hornblende. The
rock also contains trace quantities of plagioclase, biotite, apatite,
57
Table 7.
Mineral assemblages and optically determined mineral compositions from rocks at Heaven Hill •
Ma2 Unit
Sample No.
1
1
1
1
2
2
4
4-4
4-16
LP-200
HV-1
4-16B
4-16C
4-6
+
x
x
x
x
x
x
x
x
calcite
clinopyroxene
x
x
orthopyroxene
x
tremolite
hornblende
x
x
+
biotite
x
+
phlogopite
quartz
x
plagioclase
+
K-feldspar
+
+
scapolite
x
x
+
+
x
x
x
+
+
+
sphene
x
x
x
+
apatite
+
+
graphite
+
opaques
59
85
MG opx
An plag
39.5
Me scap
70
62
x- major constituent
+- minor constituent
MG
x
x
garnet
MG cpx
+
= lOO(Mg/Mg+Fe)
1 reported by Valley and Essene, 1980
+
+
65
94
55
83
51.5
46.5
100
58
zircon, opaques, garnet and orthopyroxene (MG 21). The upper contact
of this granitic gneiss is not exposed but its maximum thickness is
approximately lOOm (328').
Unit 4 consists of interlayered felsic gneisses, amphibolites and
calcareous quartzites. This unit will not be visited during the stop,
however, sample 4-6 from this unit contains a potentially useful assemblage for placing upper limits on metamorphic conditions in this area.
Sample 4-6 is a calcareous quartzite containing diopside (MG 100),
tremolite, phlogopite and quartz.
Figure 21 is a schematic T-XCO diagram for the system K.AlSi o 83
MgSi03-Si02-H2o-co2. The triangle 2 Sa-En-Vapor is projected from
Si0 2 and collapsed along H 0-co • The positions of the univariant
2
2
curves are approximate but their relative positions are governed by
Schreinemaker's rules (Zen, 1966). Univariant reactions involving both
liquid and vapor are shown as binary loops, the liquid always having the
more water rich composition. The position of the isobaric invariant
point Ph-Sa-En-Q-L-V has been experimentally determined by Wendlandt
(1981) as being near 75ooc over a wide pressure range(S to 10 kbar).
The importance of Figure 21 as regards the assemblage in 4-6 is that it
limits the assemblage Ph-Q-V to less than 7S0°c over a wide range in
pressure. Vapor absent conditions would allow higher temperatures for
this rock. The 750°c maximum is slightly below temperatures indicated
by Bohlen et al. for this area but is within the uncertainty limits
of their temperature determinations. '
One of the most interesting rocks at Heaven Hill is the biotite
gneiss. The poikiloblastic orthopyroxene in this rock apparently grew
after deformation. Buddington(l953) presented evidence that the reddish granite in the Saranac quadrangle was intrusive into both the anorthosite and metamorphosed sediments. He believed that the granite
intruded late in the deformation history of the area. Deformation must
have continued after intrusion of granite but may not have been severe
enough to destroy the textures in the biotite gneiss.
The occurrence of orthopyroxene in the granite at this locality may
be related to its proximity to the calc-silicate rocks. As shown by
Wendlandt(l981) and Figure 21, the introduction of co 2 to a granite
would tend to promote crystallization of orthopyroxene. Alternatively,
biotite+quartz may have broken down in the presence of a co 2-rich vapor
base.
Notes:
59
Stop 10.
THE 1063 MYLONITE
On the west side of Route 73, 1.3 miles (2.1 km) south of the village
of Keene Valley at BM 1063 and opposite the Beer bridge across the
Ausable River, the anorthosite is cut by a well-developed mylonitic zone
about 2'(0.6 m) wide which trends N55E35NW. Park on the west shoulder
of the road south of the outcrop and walk back. The anorthosite here
appears to be the normal gabbroic type; however, the mafic minerals occur
in clots and aggregates of augite+apatite, many of which are bent into
mini- and microfolds. These clots and the sparse megacrysts of labradorite form a foliation which trends N70W70SW. The coarse grained contaminated felsic anorthosite and the mafic clots of partially assimilated
augite+apatite represent either remnants of a Grenville phosphate-rich
calc-silicate rock, or a mafic cumulate segregated from a gabbroic anorthosite melt. The iron content of the augite, lOOFe/(Fe+Mg)= 47, while
too high for felsic anorthosite, is representative for anorthositic gabbro. Further, the profusion of "100" and "001" metamorphic pigeonite
exsolution lamellae in the host augite(Jaffe et al., 1975 and Fig. 6)
suggest that the clots may derive from anorthositic gabbro, where such
are common, rather than from a Grenville calc-silicate lithology, where
they are rare. Labradorite megacrysts in this rock are nevertheless
higher in anorthite than felsic anorthosites of the Marcy region, and
show An50.5-54.5 rather than the typical An 46 _48' suggesting a probable
assimilation of calcium from the augite-apatite-rich clots of xenoliths.
For this reason we classify such rocks ~s a percalcic subfacies of the
gabbroic anorthosite facies.
Just north of a small waterfall, the outcrop changes dramatically :
the rough foliation gives way to fine layering along which the dark
minerals occur as streaks and schlieren, though occasional megacrysts
have escaped granulation and appear as flaser, The mylonite is focused
in a 2' (0.6 m) zone which dies out gradually to the north after about
20' (6 m) giving way again to percalcic anorthosite. Just beyond a covered
interval, the north end of the outcrop contains a mafic rock, in rudely
vertical attitude, but somewhat bent about a sub-horizontal axis, perhaps
earlier than the mylonitization. It has been named "aproxite" by one
of the authors, in allusion to its bimineralic apatite+pyroxene composition, which is identical with that comprising the mafic clots in anorthosite host rock at the south end of the outcrop. The mineralogy thus suggests that the "aproxite" is a folded layer in anorthosite rather than
a mafic dike.
The mylonitic zone does not retain any of its primary magmatic or
high grade metamorphic mineralogy but is totally retrograded to a finegrained mixture of albite, prehnite, sericite, quartz, chlorite, pumpellyite,
epidote, and calcite. Labradorite is altered by the following probable
retrograde reaction:
+
4 Lab
2 Ab
+
Prehnite
Or
+ Water
+
Sericite
+ Qz
60
Augites have been drawn out into elongate lenses, spindles, and
schlieren, and totally retrograded to a mixture of fibrous, isotropic
chlorite and pumpellyite with a little calcite. Apatite, alone, remains unaltered, appearing as microflaser in the mylonitized base.
This wet assemblage is inconsistent with deep, ductile shear and
suggests that the mylonitized zone originated by brittle shear or cataclasis in a wet, relatively shallow crustal setting.
That the shear zone was initially a deep, ductile mylonite, later
retrograded, remains a possibility.
Notes:
Table 8.
Modes of host intrusive anorthositic ferrogabbro and supracrustal xenoliths at the Roaring
Brook intrusion breccia, and of ferromonzonite granulite of the summit of Giant Mt., Marcy Massif,
Adirondacks, N.Y.
Anorthositic
Ferrogabbro
Quartz
Microperthite
SuDDDit,
Giant Mt.
Grenville Supracrustal Xenoliths
RB-4
M±.!.
0
0
0
0
0
0
0
7.4
0
0
48.0
55.0
0
1.0
0
RB-4f-a
RB-4f-b
RB-4f-c
RB-4-Y
RB-4-d
Gi-25-S
1.0
27.2
Plagioclase:
Megacrysts.!/
36.0
0
0
Matrixf./
10.0
39.3
77.5
Gamet
3.0
0
0
0
0
20.0
42.0
54.0
42.8
22.0
tr
0
0
9.0
0
14.9
4.6
4.5
10.0
LO
17.0
tr
10.0
5.2
AugiteY
11.0
28.1
2.0
47.0
7.5
1.0
35.0
5.4
Orthopyroxene~/
0
Ilmenite
5.8
0
0
0
0
0
tr
2.1
Magnetite
tr
tr
0
0
0.5
tr
tr
0.4
Homblende
0
5.6
0
0
0
48.0
0
0
Biotit~/
0
22.5
10.0
0
0
0
0
0
3.2
tr
0.5
tr
tr
tr
tr
1.0
Apatite
Sphene
Totals
0
--
100.0
.!/ Mol. % An
40 - 48.5
l:_/ Mol. % An
27 - 30
]./ Mol. % Fs
71
Mol. % Fs
64
i
1
0
--
100.0
0
--
100.0
28
1.0
--
100.0
0
--
100.0
22
22
0
--
100.0
0
-100.0
0
--
100.0
32
32
23
47
63
57
74
14
Mol. % Fe
15-19
35
Avge. grain(mm) .2-.5(rnat.)
3-lO(meg.)
.12-.16
.15
Color Index
61
22
47
.15
.15
49
25
.15
.JO
58
45
.32
29
"'
1--'
62
GIANT MT.
s
N
4000 1
1220m
'
/
/
/
'
/
L)'
I
3000'
915m
/
/
/
'
/
"D
'
/
5
8
2
6
2000'
61Clm
4
3
1
Figure 22. Schematic N-S section from Putnam Brook at Highway 73
(1200', 366m) to the summit of Giant Mt. (4627', 14llm).
Ahout 1000'(305m) of multiply deformed Grenville sediments
are overlain by about 2400'(732m) of felsic volcanics
~
which are remelted by the intrusion of felsic anorthosite
o>; pyroxenite dikes
<4> intrude
Ill
D
mm
(1)
(2)
anorthosite; gabbroic anorthosite P~1I
(5) invades all,
producing block structure and the Roaring Brook breccia Iii) (6);
tectonism and shearing near the 4000'(1220m) elevation
thrust the top of Giant with possible transport to the south,
and deform gabbroic anorthosite to a gneiss l?;j} (7). A
second generation of felsic dikes ~
(8 ) cuts anorthositic rocks. Elizabethtown quadrangle.
63
Stop 11.
THE ROARING BROOK INTRUSION BRECCIA ON GIANT MOUNTAIN
From Stop 10, drive 1.6 (2.6 km) miles south on highway 73 through
the village of St. Huberts to the small parking area on the east side
of the road at the beginning of the Roaring Brook trail to the summit
of Giant Mt., elev. 4627' (1411 m). If the small lot is filled with
climbers' cars, take the next right turn off Route 73 into the larger
public parking area maintained by the Ausable Club. Secure all vehicles, as we have had our cars broken into here, and had back-packs stolen
while we were out mapping in the area.
We are at the base of Giant Mt., "the Giant of the Valley", near
the boundary(73045' W. Long.) joining the east edge of the Mt. Marcy
quadrangle with the west edge of the Elizabethtown quadrangle. We will
climb about one-third of the way up the mountain, ascending about 1000'
(305 m) in 1.5 miles (2.4 km) from elev. 1260' (384 m) at the parking
lot to the 2260' (689 m) level of the Roaring Brook intrusion breccia.
We will spend about 3 - 3 1/2 hours on the mountain and have lunch above
at the 2260' (689 m) level of Roaring Brook, where the water is potable.
DeWaard(l970) in a geologic study of the Roaring Brook intrusion
breccia concluded that, except for Grenville xenoliths, all of the rocks
exposed on Giant Mt. represent an "anorthosite-charnockite suite" differentiated from a single parent magma. This suite is considered to be
made up of all types of his proposed seq9ence including: norite, jotunite,
mangerite, farsundite, and charnockite. We discourage the usage of these
names for orthopyroxene-bearing calc-alkaline rocks whether they be of
igneous or metamorphic origin. We find no evidence of any fractionated
anorthosite-charnockite suite in the Marcy massif and see the anorthosite
and silica-poor, alkali- and iron-rich rocks as the products of crystallization of separate, unrelated melts. Further, the alkali-rich rocks show
no convincing evidence of having fractionated at all, and more likely represent individual batches of anatectic melts of crustal material. If
the alkali- and iron-rich gneisses of Pitchoff Mt., Giant Mt., and the
Big Range are compared, the Fe-content of pyroxenes shows no systematic
increase with the alkali+silica content(Tables 1 and 8).
We see the geology of Giant Mt. as a 3400' (1037 m) section (Fig. 22)
of supracrustal sedimentary and felsic volcanic rocks of probable Grenville
age, invaded first by felsic andesine anorthosite, next by pyroxenite
dikes, and then by a large volume of gabbroic-noritic anorthosite which represents the most abundant rock type on Giant Mt. Block structure, featuring
fractured blocks of felsic anorthosite broken and brecciated by later
gabbroic-noritic anorthosite magma, is widespread all over the mountain,
and can be observed in Roaring Brook and over a major portion of the Ridge
Trail from Chapel Pond to the sunnnit. During intrusion, separate batches
of silica-poor, alkali- and iron-rich melts of supracrustal material were
generated by, and partially mixed with gabbroic anorthosite magma carrying calcic andesine crystals. Several of these, es:pecially the host rock
of the Roaring Brook intrusion breccia, represent hybrid Grenville-anorthosite rocks, or gabbroic-noritic anorthosite magma contaminated with partially remelted crustal rocks and refractory xenoliths or restites of diverse lithology.
0\
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Figure 23. Schematic outcrop map at the top of Roaring Brook Falls, Giant Mt., showing blue microperthite monzonite dike with foliated margins discordantly transecting foliated gabbroic anorthosite. Mt. Marcy quadrangle.
65
Quartz-poor, alkali-rich anatectic melts may have been generated
by the presence of co -rich fluids derived from Grenville carbonates
2
in the metasedimentary sequence(Wendlandt, 1981). Although absent from
the xenolith lithologies in Roaring Brook, Grenville marbles do occur
nearby on the Ridge Trail from Chapel Pond to Giant summit. Small bodies
of ferrosyenite and aplite transgress the anorthositic rocks in Roaring
Brook and elsewhere on Giant Mt., and at the summit itself, the rock is
largely ferromonzonite(mangerite) granulite and gneiss(Table 8).
Walk one-quarter mile along the level beginning of the steep Roaring Brook trail to Giant Mt. summit. Turn left at the trail sign and
begin steep ascent to the top of Roaring Brook falls. The spectacular
waterfalls visible from highway 73 are formed in an E-W shear zone parallel to Roaring Brook, in which a diabase dike is contained. The shatter
zone and the dike have contributed to the differential erosion and
development of the falls. About one-half mile of climbing brings us
to the top of the falls where there are fine outcrops and an excellent
view south and southwest to the high peaks of the Great Range. Here
the country rock is a well-foliated gabbroic-noritic anorthosite(C.I.=
10-12 and M.I.=30). Augites are rimmed with hornblende and a relatively
high Mg/Fe ratio has inhibited the formation of garnet. Calcic andesine
megacrysts define a steeply dipping N 20-38 W 60-82 NE foliation crossing
the brook. In the brook, and parallel to it, is an equigranular augite
monzonite dike carrying blue-gray microperthite, easy to confuse with
the more common blue-gray calcic andesine megacrys.ts of the anorthosites.
The monzonite carries irregular xenoliths of felsic andesine anorthosite
such that the entire rock may be readily misidentified as anorthosite.
The margins of the monzonitic dike are very strongly foliated parallel
to the brook and contain exactly the same minerals found in the equigranular dike. The dike and its foliate margins sharply cross-cut the gabbroic
anorthosite country rock(Fig. 23).
We will probably not take time to climb up the brook, where several
hypersthene pyroxenite and gabbro dikes cross-cut felsic anorthosite,
then were broken and invaded by gabbroic anorthosite, the prevalent anorthositic lithology of Giant Mt.
We return to the Roaring Brook trail and another climb of about onequarter to one half mile(O. 8km) brings us to a brook crossing. We will
first make a stop near here to study a large anorthosite erratic which
exhibits a variety of anorthositic textures in complex relationship.
Cross the brook carefully, turn left at the trail junction to Giant's
Washbowl and Nubble, and continue the climb. After we cross two brooks,
our climb bring$ us to the 2260'(689 m) level where a small path through
the brush to the left brings us out on the open banks and rock pavements
of the spectacular intrusion breccia of Roaring Brook.
About 40% of the volume of the pavement outcrops consist of xenoliths
of varied size and shape(Figs. 24,25) enclosed in a host metamorphosed
intrusive rock best described as a garnetiferous anorthositic ferrogabbro.
It is a hybrid rock made up of broken, partly comminuted, highly garnetized crystals of calcic andesine, An40-48.5' and mafic clots with inverted
66
i)
••
~
~
~
ll,J
~
.._
<:
C)
V)
.._~
<::
•,
--------
·:•·•••
•••
)..._
<::
~
.....,
.._
(.:)
~
<::
Cl)
~
@
~
ct:
~
Q:
..•.
o•
0
.. "
•
j
•
""\:.
Figure 24(above) and 25(below). Detailed geologic map of upper zone of Roaring Brook intrusl.on
breccia showing profusion of xenoliths of metamorphosed sedimentary and volcanic (Grenville?)
origin. Smallest xenoliths shown are about 15 cm(6") in diameter. Elizabethtown quadrangle.
'
-=&1
.. ,
• ..,
••
'
°'......
0\
00
N
F"iviol }ell,, f 'K2-
m
•••
,.
'00~
&>..•~fl
\')
'
'.:I
V'
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3
4
5
I
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2
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'
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.,
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•
.
-~
•
'\~~
~~
Figure 26. Detail of felsic anorthosite xenoliths in anorthositic ferrogabbro-ferrodiorite
matri:x near contact zone of intrusion breccia and anorthosita,Roaring Brook, elevation
2050' (625m), Giant Mt. at Elizabethtown-Mt. Marcy quadrangle boundary.
-.................. ..
'·
"·~
o~
-..\
ttS
-
\'
""
.
\.
ff''~
~
e
N
I NTRUSI ON BRECCIA :
ROARING BROOK.
GIANT MT
NY.
m
I
)lc--:1
0
2
I
3
4
5
+---+-~
Figure 27. Contact zone of Roaring Brook intrusion breccia with a large block of felsic andesine
anorthosite near the 2000'(610m) elevation, Roaring Brook, Giant Mt., Mt. Marcy quadrangle.
°'
l.O
70
pigeonite,
now eulite, Fsj 0_ 75 , rimmed by recrystallized augite, all
in a matrix of sodic andesine, An27-30, ilmenite, minor magnetite and
apatite, and variable amounts of microperthite(Table 8). Although the
pink-weathering mesocratic outcrop surfaces suggest a mafic syenite,
the host rock is generally low in alkali feldspar, and the color is
caused by the decomposition of the iron-rich pyroxenes. The microscopic
texture is metamorphic and granoblastic, dominated by abraded calcic
andesine megacrysts extensively replaced by chains and microporphyroblasts
of euhedral garnet, and ferroaugite containing copious amounts of metamorphic "100" and "001" pigeonite exsolution lamellae. The microfabric,
thus, in no way exhibits the magmatic texture anticipated from the outcrop appearance.
A very detailed geologic map on a scale of 1:76 made by John Findley
in the sununer of 1982(U. Mass. Dep't. of Geol. & Geog. Senior Honors
Thesis(l983) in preparation), shows xenoliths as small as about 15 cm
(6") length(Figs. 24,25). The detailed mapping shows that xenoliths
make up a much greater volume of the outcrops than would be estimated
from cursory examination, and reveal a complex history. There was polyphase deformation of supracrustal rocks, intrusion of felsic andesine
anorthosite with associated contact metamorphism, brecciation accompanying the intrusion of anorthositic norite, partial anatexis and chemical
exchange between supracrustal material and magma, and regional metamorphism converting the anorthositic norite intrusive into a garnetiferous
anorthositic ferrogabbro. The conversion of anorthositic norite to
garnet-augite-rich anorthositic metagabbro would use up much of the hypersthene, ilmenite, and anorthite components of the former.
Many of the xenoliths lie in parallel orientation, and dip gently
northward, possibly in their pre-intrusion orientation; others have been
rotated during brecciation accompanying intrusion. A very important result of the geological mapping by the authors reveals that supracrustal
layering is persistently subhorizontal(gently north- gently south-dipping)
over 3400'(1037 m) of section, ranging from Putnam Brook migmatitic outcrops at 1300'(396 m) through the Roaring Brook intrusion breccia at
2250'(636 m) in the gabbroic anorthosite of the Ridge Trail, and on the
summit of Giant Mt. itself where a layered ferromonzonite granulite dips
12°N to l0°s. This pervasive subhorizontal trend of supracrustals stands
in marked contrast to the equally pervasive steeply dipping foliation
of the anorthositic rocks. We interpret this to mean that the anorthosite has discordantly intruded the supracrustals.
Anorther interesting feature of the Roaring Brook supracrustal xenoliths is their texture. They most commonly occur as fine-grained, 0.10.3 mm, granulites, often layered but s.eldom gneissic. Such texture
may be a relic of contact metamorphism during which hornfelses were
formed. Many of the xenoliths contain magnesian pyroxenes and biotite
accompanied by sodic plagioclase and microperthite, whereas the anorthositic ferrogabbro intrusive host carries ferroan pyroxenes, more calcic
plagioclase, and little potassium feldspar. Common reaction borders of
ferroaugite on xenoliths, and dikes and tongues of syenitic material,
suggest that evolution of anatectic felsic melts and assimilation of
iron and potassium by the invading anorthositic melts were important processes.
71
As we descend the brook, occasional xenoliths of felsic andesine
anorthosite appear in the company of the pyroxene granulite xenoliths
(Fig. 26) and the former become larger and more abundant until a contact
zone between felsic anorthosite and the intrusion breccia(Fig. 27) is
reached at an elevation close to 2000'(610 m) in the easternmost part
of the Mt. Marcy quadrangle. The contact is irregular and the felsic
anorthosite xenoliths in the intrusion breccia are interpreted to be
remnants of the earlier anorthosite intrusion, brecciated and caught
up in the later noritic-gabbroic ano~rthosite magma; they are block
structure remnants. De Waard classed the invading rock as jotunite
(hypersthene monzodiorite), Kemp (1921) called it anorthosite, and we
prefer the name anorthositic ferrogabbro. Although the plagioclase now
, the An content has been considerably reduced because
shows An _
27 48 5
of the copious metamorphic growth of garnet and augite. Thus, anorthositic ferrogabbro or ferrodiorite are equally appropriate.
We will return to the top of the intrusion breccia for lunch and discussion. Descent to the parking area will take about forty minutes.
Time permitting, we will make a final stop of three hour duration to
the "Chicken Fold" of Rooster Comb Mt. with those who have time to
remain.
Notes:
•
72
Stop 12.
THE CHICKEN FOLD OF ROOSTER COMB MOUNTAIN, RC-1
Drive north on Route 73 past the village of St. Huberts and park
just north of the Ausable River bridge on either side of the road. On
the west side of Route 73, walk up a private road, bearing left around
the curve at the top of the first hill, past a house on the left, and
find a trail west into the woods, where the road curves right. Follow
this trail west about 3/4 mile (1.2 km) almost to a waterfall, where a
trail to Snow Mt. bears right(north). Further along, the trail to Snow
Mt. summit will again bear right. Avoid it and continue straight ahead
(approximately northwest) to the intersection of the Flume Brook trail
to Rooster Comb. Turn left(west) and climb another 1/4 mile(0.4 km) to a
large outcrop about 500 feet (152 m) NW of trail.
In this spectacular outcrop, which forms a small knob about 500 x
1500' (152 x 456 m) with its long axis SSW-NNE just east of and below
the sunnnit of Rooster Comb Mt., a gray, lineated and foliated percalcic
anorthosite is intimately infolded with pink alaskite and buff-weathering scapolite gneiss (Table 9). Quartz in the alas kite is. strongly elongated and bent into micro-folds. The deep green clinopyroxene is ironrich, lOOFe/(Fe+Mg) = 60, contains abundant inclusions of sphene, and is
partially retrograded to hornblende. The hedenbergitic clinopyroxene
with sphene inclusions is inherited Grenville calc-silicate.
The fold axis is parallel to a strong augite+hornblende lineation in
a recline fold section is N 22-40 E 13-18° and the overall foliation
strikes N 50-78 Wand dips 12-40 NE. Over the 75' section, beds differ
in thickness from fractions of inches (cm) to several feet (m). There
has been a great deal of "bedding plane" slip, as it is difficult to
trace a layer from one fold into another(Fig. 28). Higher up in the outcrop, the interlayered alaskite and anorthosite are thicker.
A severely brecciated layer of anorthosite lies on top of the knob.
This brecciation is later than the folding although at one end of the
knob, small scale folds in the anorthosite are quite unlike the larger
folds below, and are perhaps related to the brecciation. An alternative
and not necessarily contradictory hypothesis is that these are isoclinal
recumbent folds that have simply been refolded during later faulting
similar to the Baxter fold at Beede ledge. Their dissimilarity to the
larger folds below could be explained by a difference in rock type or
thickness of original beds.
Whatever the origin of the folds, they are significant because
here the anorthosite is clearly involved in the east-west isoclinal recumbent folding we have seen wherever the Grenville beds have not been
disrupted by faulting. The scapolite gneiss and the alaskite were contact-metamorphosed by an early intrusion of anorthosite, probably as sills
or lit-par-lit; the whole package was then isoclinally and recumbently
folded along an east-west axis, and subsequently faulted and sheared,
probably repeatedly.
'' 6 ed d i ri 9
p I0.11 ~ aslip
The Folds o.t
Rooster Comb
r ltl.ne.
,,
sl~,.)I
.
0
0
1°
5:
3'
Figure 28. Complex recumbent folding of intercalated percalcic anorthosite, alaskite, and
scapolite gneiss in the Chicken Fold of Rooster Comb Mt., Mt. Marcy quadrangle.
-...J
w
74
Table 9.
Modes of percalcic anorthosite, alaskite, and scapolite
gneiss from the Chicken Fold on Rooster Comb Mt.
Percalcic
Anorthosite Gn
RC-1-a
Alaskite Gn
RC-1-g
Quartz
0
34.5
Microcline
0
56.5
Albite
Scapolite Gn
RC-1-s
9.0
Andesine
84.3
40
Scapolite
3.5
57
Clinopyroxene
7.0
Hornblende
4.0
Pyrite
0
Sphene
1.2
Total
c. I.
Plagioclase:
Megacrysts - An48.5
Matrix
An44.5
Clinopyroxene-Fs
60
tr
3
100.0
100.0
100.0
16
<0.5
3
Alkali feldspar:
unmixed microperthite
75
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~~~~~~~~
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....-.
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-- ---
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-------
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0
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~~~~~~~-
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'
'i
~
U
.
Hea v en
·· ·
Hill
j
='·j
9-:-B
=f'i' -
.
·(
~::::Ea:::::J=i::=c='3:::i:==:==:==:==:==:==:=i:======:::::::iE==:==:==:==:==:==:i::::======~'
3000
F3
0
E"'
F3
E-3
3000
E"'+3
6000
e----3
E+3
9000
e--=-+3
12000
E+3
e+3
1500C
e---3
18000
E+3
E±3
21000 FEET
F+3
MllES
Plate 4. Parts of U.S.G.S. topographic base maps of the 15'
Santanoni(l953}, 15' Mt. Marcy(l953} and Elizabethtown
quadrangles with superposed trails in the Adirondack
High Peaks Region, prepared by the Adirondack Mountain
Club, Inc.(1980). Grenville Club 1983 field trip stops
are numbered 1-12.